A mutation in Rab27a causes the vesicle transport defects observed in ashen mice

Griscelli syndrome: characterization of a new mutation and rescue of T-cytotoxic activity by retroviral transfer of RAB27A gene

Griscelli syndrome (GS) is caused by mutations in the MYO5A (GS1), RAB27A (GS2), or MLPH (GS3) genes, all of which lead to a similar pigmentary dilution. In addition, GS1 patients show primary neurological impairment, whereas GS2 patients present immunodeficiency and periods of lymphocyte proliferation and activation, leading to their infiltration in many organs, such as the nervous system, causing secondary neurological damage. We report the diagnosis of GS2 in a 4-year-old child with haemophagocytic syndrome, immunodeficiency, and secondary neurological disorders. Typical melanosome accumulation was found in skin melanocytes and pigment clumps were observed in hair shafts. Two heterozygous mutant alleles of the RAB27A gene were found, a C-T transition (C352T) that leads to Q118stop and a G-C transversion on the exon 5 splicing donor site (G467+1C). Functional assays showed increased cellular activation and decreased cytotoxic activity of NK and CD8+ T cells, associated with defective lytic granules release. Myosin-Va expression and localization in the patient lymphocytes were also analyzed. Most importantly, we show that cytotoxic activity of the patient's CD8+ T lymphocytes can be rescued in vitro by RAB27A gene transfer mediated by a recombinant retroviral vector, a first step towards a potential treatment of the acute phase of GS2 by RAB27A transduced lymphocytes.

Rab8 regulates the actin-based movement of melanosomes

Rab GTPases have been implicated in the regulation of specific microtubule- and actin-based motor proteins. We devised an in vitro motility assay reconstituting the movement of melanosomes on actin bundles in the presence of ATP to investigate the role of Rab proteins in the actin-dependent movement of melanosomes. Using this assay, we confirmed that Rab27 is required for the actin-dependent movement of melanosomes, and we showed that a second Rab protein, Rab8, also regulates this movement. Rab8 was partially associated with mature melanosomes. Expression of Rab8Q67L perturbed the cellular distribution and increased the frequency of microtubule-independent movement of melanosomes in vivo. Furthermore, anti-Rab8 antibodies decreased the number of melanosomes moving in vitro on actin bundles, whereas melanosomes isolated from cells expressing Rab8Q67L exhibited 70% more movements than wild-type melanosomes. Together, our observations suggest that Rab8 is involved in regulating the actin-dependent movement of melanosomes.

Rab27a mediates the tight docking of insulin granules onto the plasma membrane during glucose stimulation

The monomeric small GTPase Rab27a is specifically localized on both secretory granules and lysosome-related organelles. Although natural mutations of the Rab27a gene in human Griscelli syndrome and in ashen mice cause partial albinism and immunodeficiency reflecting the dysfunction of lysosome-related organelles, phenotypes resulting from the defective exocytosis of secretory granules have not been reported. To explore the roles of Rab27a in secretory granules, we analyzed insulin secretion profiles in ashen mice. Ashen mice showed glucose intolerance after a glucose load without signs of insulin resistance in peripheral tissues or insulin deficiency in the pancreas. Insulin secretion from isolated islets was decreased specifically in response to high glucose concentrations but not other nonphysiological secretagogues such as high K+ concentrations, forskolin, or phorbol ester. Neither the intracellular Ca2+ concentration nor the dynamics of fusion pore opening after glucose stimulation were altered. There were, however, marked reductions in the exocytosis from insulin granules predocked on the plasma membrane and in the replenishment of docked granules during glucose stimulation. These results provide the first genetic evidence to our knowledge for the role of Rab27a in the exocytosis of secretory granules and suggest that the Rab27a/effector system mediates glucose-specific signals for the exocytosis of insulin granules in pancreatic beta cells.

Molecular defects that affect platelet dense granules

Platelet dense granules form using mechanisms shared by melanosomes in melanocytes and by subsets of lysosomes in more generalized cells. Consequently, disorders of platelet dense granules can reveal how organelles form and move within cells. Models for the study of new vesicle formation include isolated delta-storage pool deficiency, combined alphadelta-storage pool deficiency, Hermansky-Pudlak syndrome (HPS), Chediak-Higashi syndrome, Griscelli syndrome, thrombocytopenia absent radii syndrome, and Wiskott-Aldrich syndrome. The molecular bases of dense granule deficiency are known for the seven subtypes of HPS, as well as for Chediak-Higashi syndrome, Griscelli syndrome, and Wiskott-Aldrich syndrome. The gene products involved in these disorders help elucidate the generalized process of the formation of vesicles from extant membranes such as the Golgi.

Melanocyte-specific proteins are aberrantly trafficked in melanocytes of Hermansky-Pudlak syndrome-type 3

Hermansky-Pudlak Syndrome-type 3 (HPS-3) is a relatively mild subtype of HPS with minimal cutaneous and ocular depigmentation. The HPS-3 gene encodes a novel protein of unknown function with a predicted molecular weight of 114 kd. To assess the role of the HPS3 protein in melanization, cultured melanocytes developed from HPS-3 patients were evaluated biochemically and histologically for activity and localization of melanocyte-specific proteins. Endogenous tyrosinase activity of HPS-3 melanocytes was substantial, but tyrosinase activity and melanin synthesis was suppressed in intact melanocytes. However, the level of suppression, as well as extent to which up-regulation by isobutylmethylxanthine and cholera toxin was muted, was less that in HPS-1 melanocytes. Ultrastructurally, HPS-3 melanocytes contained morphologically normal melanosomes, predominantly of stage I and II with minimal stage III and few stage IV melanosomes. Dihydroxyphenylalanine (DOPA) histochemistry demonstrated an increase in melanization of melanosomes. Unique to HPS-3 melanocytes were numerous DOPA-positive 50-nm vesicles and tubular elements present throughout the cell body and dendrites. Tyrosinase, tyrosinase-related protein-1 (Tyrp1), dopachrome tautomerase (Dct), and LAMP1 and 3 localization in HPS-3 melanocytes, as evaluated by immunocytochemistry and confocal microscopy, demonstrated a fine, floccular distribution in contrast to the coarse, granular distribution characteristic of control melanocytes. The localization profile of other proteins expressed by melanocytes (ie, Silver/Pmel17, Melan-A/MART-1, LAMP2, Rab 27, transferrin, c-kit, adaptin-3, and the HPS1 protein) appeared normal. These results suggest that a specific subset of melanocyte proteins are aberrantly trafficked throughout the HPS-3 melanocyte and may be responsible for the reduction in melanin synthesis.

Perforin and lymphohistiocytic proliferative disorders

Perforin is critical for cytotoxicity mediated by granules present in natural killer (NK) cells and cytotoxic T lymphocytes (CTLs). Perforin-deficient mice have impaired cytotoxicity by NK cells and CTLs, resulting in failure to control infections with certain viruses or bacteria. Infection of perforin-deficient mice with lymphocytic choriomeningitis virus results in haemophagocytic lymphohistiocytosis and elevated levels of pro-inflammatory cytokines. Mutations throughout the perforin gene have been identified in patients with familial haemophagocytic lymphohistiocytosis (FHL) type 2. These patients present with fever, hepatosplenomegaly, pancytopenia, have marked elevations of T-helper type 1 and type 2 cytokines, and have impaired NK cell and CTL cytotoxicity. A number of infectious pathogens have been implicated as triggering the onset of disease. Identification of mutations in perforin as the cause of FHL should allow prenatal diagnosis of the disorder. While stem cell transplantation is curative, gene therapy might be effective in the future.

Mechanisms of polarized growth and organelle segregation in yeast

Cell polarity, as reflected by polarized growth and organelle segregation during cell division in yeast, appears to follow a simple hierarchy. On the basis of physical cues from previous cell cycles or stochastic processes, yeast cells select a site for bud emergence that also defines the axis of cell division. Once polarity is established, rho protein-based signal pathways set up a polarized cytoskeleton by activating localized formins to nucleate and assemble polarized actin cables. These serve as tracks for the transport of secretory vesicles, the segregation of the trans Golgi network, the vacuole, peroxisomes, endoplasmic reticulum, mRNAs for cell fate determination, and microtubules that orient the nucleus in preparation for mitosis, all by myosin-Vs encoded by the MYO2 and MYO4 genes. Most of the proteins participating in these processes in yeast are conserved throughout the kingdoms of life, so the emerging models are likely to be generally applicable. Indeed, several parallels to cellular organization in animals are evident.

Purification and Functional Analysis of a Rab27 Effector Munc13‐4 Using a Semiintact Platelet Dense‐Granule Secretion Assay

We have demonstrated that small GTPase Rab27 regulates dense-granule secretion in platelets. Using Rab27a affinity chromatography, we purified Munc 13-4 as a novel Rab27a interacting protein from platelet cytosol. This chapter describes the purification of Munc 13-4 and an in vitro assay system analyzing the mechanism of dense-granule secretion in platelets. The activity of Munc 13-4 is tested in this assay.

Expression and characterization of Rab38, a new member of the Rab small G protein family

Rab38 is a new member of the Rab small G protein family that regulates intracellular vesicle trafficking. Rab38 is expressed in melanocytes and it has been clarified that a point mutation in the postulated GTP-binding domain of Rab38 is the gene responsible for oculocutaneous albinism in chocolate mice. However, basic information regarding recombinant protein production, intracellular location, and tissue-specific expression pattern has not yet been reported. We produced recombinant Rab38 using a baculovirus/insect cell-protein expression system. A combination of Triton X-114 phase separation and nickel-affinity chromatography yielded exclusively prenylated Rab38 that bound [alpha-32P]-GTP. The mRNA and the native protein were expressed in a tissue-specific manner, e.g., in the lung, skin, stomach, liver, and kidney. Freshly isolated rat alveolar type II cells were highly positive for the mRNA signal, but the signal was rapidly lost over time. Immunofluorescence staining demonstrated that expressed GST-tagged Rab38 was mainly co-localized with endoplasmic reticulum-resident protein and also partly with intermittent vesicles between the endoplasmic reticulum and the Golgi complex. These results indicate that Rab38 is expressed non-ubiquitously in specific tissues and regulates early vesicle transport relating to the endoplasmic reticulum, and hence suggest that Rab38 abnormality may cause multiple organ diseases as well as oculocutaneous albinism.

Involvement of Rab27 in antigen-induced histamine release from rat basophilic leukemia 2H3 cells

The Rab family small G proteins regulate discrete steps in vesicular transport pathways. Recent studies indicate that one member of the Rab family, Rab27A, regulates the transport of lysosome-related organelles, such as melanosome distribution in melanocytes, lytic granule release in cytotoxic T cells, and dense granule release in platelets. Here, we have examined the involvement of Rab27A in the exocytic transport of another lysosome-related organelle, the basophilic secretory granule, in basophils. We have found that Rab27A locates on basophilic secretory granules containing histamine in rat basophilic leukemia (RBL) 2H3 cells. In addition, exogenous expression of dominant active Rab27A reduces antigen-induced histamine release from the cells. We have moreover identified Munc13-4 as a Rab27A target using a CytoTrap system and found that exogenous expression of Munc13-4 affects antigen-induced histamine release from RBL-2H3 cells. These results demonstrate that Rab27A plays a crucial role in antigen-induced histamine release from RBL-2H3 cells.

Rab GTPases and myosin motors in organelle motility

The actin cytoskeleton is essential to ensure the proper location of, and communication between, intracellular organelles. Some actin-based myosin motors have been implicated in this process, particularly members of the class V myosins. We discuss here the emerging role of the Ras-like GTPases of the Rab family as regulators of myosin function in organelle transport. Evidence from yeast secretory vesicles and mitochondria, and mammalian melanosomes and endosomes suggests that Rab GTPases are crucial components of the myosin organelle receptor machinery. Better understood is the case of the melanosome where Rab27a recruits a specific effector called melanophilin, which in turn binds myosin Va. The presence of a linker protein between a Rab and a myosin may represent a general mechanism. We argue that Rabs are ideally suited to perform this role as they are exquisite organelle markers. Furthermore, the molecular switch property of Rabs may enable them to regulate the timing of the myosin association with the target organelle.

Role of myosin VIIa and Rab27a in the motility and localization of RPE melanosomes

Myosin VIIa functions in the outer retina, and loss of this function causes human blindness in Usher syndrome type 1B (USH1B). In mice with mutant Myo7a, melanosomes in the retinal pigmented epithelium (RPE) are distributed abnormally. In this investigation we detected many proteins in RPE cells that could potentially participate in melanosome transport, but of those tested, only myosin VIIa and Rab27a were found to be required for normal distribution. Two other expressed proteins, melanophilin and myosin Va, both of which are required for normal melanosome distribution in melanocytes, were not required in RPE, despite the association of myosin Va with the RPE melanosome fraction. Both myosin VIIa and myosin Va were immunodetected broadly in sections of the RPE, overlapping with a region of apical filamentous actin. Some 70-80% of the myosin VIIa in RPE cells was detected on melanosome membranes by both subcellular fractionation of RPE cells and quantitative immunoelectron microscopy, consistent with a role for myosin VIIa in melanosome motility. Time-lapse microscopy of melanosomes in primary cultures of mouse RPE cells demonstrated that the melanosomes move in a saltatory manner, interrupting slow movements with short bursts of rapid movement (>1 RR01183m/second). In RPE cells from Myo7a-null mice, both the slow and rapid movements still occurred, except that more melanosomes underwent rapid movements, and each movement extended approximately five times longer (and further). Hence, our studies demonstrate the presence of many potential effectors of melanosome motility and localization in the RPE, with a specific requirement for Rab27a and myosin VIIa, which function by transporting and constraining melanosomes within a region of filamentous actin. The presence of two distinct melanosome velocities in both control and Myo7a-null RPE cells suggests the involvement of at least two motors other than myosin VIIa in melanosome motility, most probably, a microtubule motor and myosin Va.

dsu functions in a MYO5A-independent pathway to suppress the coat color of dilute mice

MYO5A is a major actin-based vesicle transport motor that binds to one of its cargos, the melanosome, by means of a RAB27A/MLPH receptor. When one of the members of this receptor-motor complex is mutated, the melanosomes clump in the perinuclear region of the melanocyte and are transferred unevenly to the developing hair, leading to a dilution of coat color. Mutation of a fourth gene, dilute suppressor (dsu), suppresses this coat color dilution. MYO5A is required for the peripheral accumulation of melanosomes in melanocytes, but its role in melanosome transfer to neighboring keratinocytes and the hair is unknown. Here, we show that MYO5A is nonessential for melanosome transfer, although pigment incorporation into the hair in MYO5A-deficient mice is uneven, probably due to the clumping of melanosomes that occurs in the perinuclear region of mutant melanocytes. We also show that dsu is caused by a loss-of-function mutation in a unique vertebrate-specific protein that appears to function in an MYO5A-independent pathway to alter pigment incorporation into the hair. Therefore, dsu identifies a unique protein involved in pigmentation of the mammalian hair.

Rab27A-binding protein Slp2-a is required for peripheral melanosome distribution and elongated cell shape in melanocytes

The synaptotagmin-like protein (Slp) family is implicated in regulating Rab27A-mediated membrane transport, but how it might do this is unknown. Here we report that Slp2-a, a previously uncharacterized Rab27A-binding protein in melanocytes, controls melanosome distribution in the cell periphery and regulates the morphology of melanocytes. Slp2-a is the most abundantly expressed of the Slp- and Slac2-family proteins in melanocytes and colocalizes with Rab27A on melanosomes. Knockdown of endogenous Slp2-a protein by small-interfering RNAs (siRNAs) markedly reduced the number of melanosomes in the cell periphery of mouse melanocytes ('peripheral dilution'). Expression of siRNA-resistant Slp2-a (Slp2-a(SR)) rescued the peripheral dilution of melanosomes induced by Slp2-a siRNAs, but Slp2-a(SR) mutants, which failed to interact with either phospholipids or Rab27A, did not. Loss of Slp2-a protein also induced a change in melanocyte morphology, from their normal elongated shape to a more rounded shape, which depended on the phospholipid-binding activity of Slp2-a, but not on its Rab27A-binding activity. By contrast, knockdown of Slac2-a (also called melanophilin), another Rab27A-binding protein in melanocytes, caused perinuclear aggregation of melanosomes alone without altering cell shape. These results reveal the differential and sequential roles of Rab27A-binding proteins in melanosome transport in melanocytes.

Rab27b is Up-Regulated in Human Griscelli Syndrome Type II Melanocytes and Linked to the Actin Cytoskeleton via Exon F-Myosin Va Transcripts

Patients with the autosomal recessive Griscelli-Pruniéras syndrome type II are immunologically impaired and have an unusual silvery-grey hypopigmented colour of scalp hair, eyelashes and eyebrows but no noteworthy pigmentary abnormalities of the skin. In most Griscelli patients, the RAB27A gene, which encodes a small GTPase that is associated with the melanosome membrane in melanocytes, is mutated. Here we discuss a genomic RAB27A deletion found in a 21-month-old Moroccan Griscelli patient. Additionally, we provide evidence that the loss of functional Rab27a in melanocytes of this Griscelli patient is partially compensated by the up-regulation of Rab27b, a homologue of Rab27a. By real-time quantitative PCR and western blot analysis, we found that Rab27b mRNA and protein, expressed at low levels in normal human melanocytes, is significantly up-regulated in melanocytes derived from this patient. Our immunofluorescence and yeast two-hybrid screening studies reveal that Rab27b can form a tripartite complex on the melanosome membrane with Melanophilin, a Rab27a effector, and protein products of Myosin Va transcripts that contain exon F. Our data suggest that up-regulated Rab27b in melanocytes of the Griscelli patient can partially take over the function of Rab27a, which could explain the fact that this patient had an evenly pigmented skin and was able to tan.

Melanosome transfer to and translocation in the keratinocyte

Complexion coloration in humans is primarily regulated by the amount and type of melanin synthesized by the epidermal melanocyte. However, additional and equally contributing factors consist of (1) efficient transfer of melanin from the melanocytes to the neighboring keratinocytes and (2) distribution and degradation of the transferred melanosomes by the recipient keratinocytes. Once synthesized in the cell body of the epidermal melanocyte, pigmented melanosomes are translocated down the dendrites and captured at the dendritic tips via various cytoskeletal elements. Molecules recently identified that participate in this process consist of Rab27a, myosin-Va and melanophilin. Eventually, these peripherally localized melanosomes are transferred to keratinocytes by a presently undefined mechanism. The protease-activated receptor-2 (PAR-2) and unidentified surface lectins and glycoproteins facilitate this transfer process. Once incorporated into the keratinocytes, melanosomes are distributed individually or as clusters, aggregated towards the apical pole of the nucleus, and degraded as the keratinocytes undergo terminal differentiation and desquamation. Ultraviolet irradiation (UVR) can modulate the process of melanosome transfer from the melanocytes to the keratinocytes. UVR can upregulate expression of PAR-2 and lectin-binding receptors and increase phagocytic activity of cultured keratinocytes. Therefore, many cellular and molecular events that occur after melanogenesis contribute to skin color.

Murine Hermansky-Pudlak syndrome genes: regulators of lysosome-related organelles

In the mouse, at least 16 genes regulate vesicle trafficking to specialized lysosome-related organelles, including platelet dense granules and melanosomes. Fourteen of these genes have been identified by positional cloning. All 16 mouse mutants are models for the genetically heterogeneous human disease, Hermansky-Pudlak Syndrome (HPS). Five HPS genes encode known vesicle trafficking proteins. Nine genes are novel, are found only in higher eukaryotes and encode members of three protein complexes termed BLOCs (Biogenesis of Lysosome-related Organelles Complexes). Mutations in murine HPS genes, which encode protein co-members of BLOCs, produce essentially identical phenotypes. In addition to their well-known effects on pigmentation, platelet function and lysosome secretion, HPS genes control a wide range of physiological processes including immune recognition, neuronal functions and lung surfactant trafficking. Studies of the molecular functions of HPS proteins will reveal important details of vesicle trafficking and may lead to therapies for HPS.

Linking albinism and immunity: the secrets of secretory lysosomes

Lysosomes are membrane-bound organelles that are found in all mammalian cells and contain hydrolases and lipases required for protein and membrane degradation. In many cells of the immune system, lysosomes also contain secretory proteins that can be released by regulated exocytosis in response to an external stimulus, providing different cell types with a wide range of effector functions. Melanosomes also use a lysosome-related organelle to secrete melanin for pigmentation. Links between albinism and immunity in patients have uncovered a number of key proteins required for lysosomal secretion and have revealed a versatile secretory mechanism that can be fine-tuned by distinct interactions in different cell types.

Role of NF-kappa B in cell survival and transcription of latent membrane protein 1-expressing or Epstein-Barr virus latency III-infected cells

Epstein-Barr virus (EBV) latency III infection converts B lymphocytes into lymphoblastoid cell lines (LCLs) by expressing EBV nuclear and membrane proteins, EBNAs, and latent membrane proteins (LMPs), which regulate transcription through Notch and tumor necrosis factor receptor pathways. The role of NF-kappa B in LMP1 and overall EBV latency III transcriptional effects was investigated by treating LCLs with BAY11-7082 (BAY11). BAY11 rapidly and irreversibly inhibited NF-kappa B, decreased mitochondrial membrane potential, induced apoptosis, and altered LCL gene expression. BAY11 effects were similar to those of an NF-kappa B inhibitor, Delta N-I kappa B alpha, in effecting decreased JNK1 expression and in microarray analyses. More than 80% of array elements that decreased with Delta N-I kappa B alpha expression decreased with BAY11 treatment. Newly identified NF-kappa B-induced, LMP1-induced, and EBV-induced genes included pleckstrin, Jun-B, c-FLIP, CIP4, and I kappa B epsilon. Of 776 significantly changed array elements, 134 were fourfold upregulated in EBV latency III, and 74 were fourfold upregulated with LMP1 expression alone, whereas only 28 were more than fourfold downregulated by EBV latency III. EBV latency III-regulated gene products mediate cell migration (EBI2, CCR7, RGS1, RANTES, MIP1 alpha, MIP1 beta, CXCR5, and RGS13), antigen presentation (major histocompatibility complex proteins and JAW1), mitogen-activated protein kinase pathway (DUSP5 and p62Dok), and interferon (IFN) signaling (IFN-gamma R alpha, IRF-4, and STAT1). Comparison of EBV latency III LCL gene expression to immunoglobulin M (IgM)-stimulated B cells, germinal-center B cells, and germinal-center-derived lymphomas clustered LCLs with IgM-stimulated B cells separately from germinal-center cells or germinal-center lymphoma cells. Expression of IRF-2, AIM1, ASK1, SNF2L2, and components of IFN signaling pathways further distinguished EBV latency III-infected B cells from IgM-stimulated or germinal-center B cells.

Rabphilin and Noc2 are recruited to dense-core vesicles through specific interaction with Rab27A in PC12 cells

Rabphilin and Noc2 were originally described as Rab3A effector proteins involved in the regulation of secretory vesicle exocytosis, however, recently both proteins have been shown to bind Rab27A in vitro in preference to Rab3A (Fukuda, M. (2003) J. Biol. Chem. 278, 15373-15380), suggesting that Rab3A is not their major ligand in vivo. In the present study we showed by means of deletion and mutation analyses that rabphilin and Noc2 are recruited to dense-core vesicles through specific interaction with Rab27A, not with Rab3A, in PC12 cells. Rab3A binding-defective mutants of rabphilin(E50A) and Noc2(E51A) were still localized in the distal portion of the neurites (where dense-core vesicles had accumulated) in nerve growth factor-differentiated PC12 cells, the same as the wild-type proteins, whereas Rab27A binding-defective mutants of rabphilin(E50A/I54A) and Noc2(E51A/I55A) were present throughout the cytosol. We further showed that expression of the wild-type or the E50A mutant of rabphilin-RBD, but not the E50A/I54A mutant of rabphilin-RBD, significantly inhibited high KCl-dependent neuropeptide Y secretion by PC12 cells. We also found that rabphilin and its binding partner, Rab27 have been highly conserved during evolution (from nematoda to humans) and that Caenorhabditis elegans and Drosophila rabphilin (ce/dm-rabphilin) specifically interact with ce/dm-Rab27, but not with ce/dm-Rab3 or ce/dm-Rab8, suggesting that rabphilin functions as a Rab27 effector across phylogeny. Based on these findings, we propose that the N-terminal Rab binding domain of rabphilin and Noc2 be referred to as "RBD27 (Rab binding domain for Rab27)", the same as the synaptotagmin-like protein homology domain (SHD) of Slac2-a/melanophilin.

The small GTPase Rab27B regulates amylase release from rat parotid acinar cells

Small GTPase Rab is a large family of putative membrane trafficking proteins, and each member is thought to regulate a specific type(s) of membrane trafficking. However, little is known about the involvement of Rab protein(s) in secretory granule exocytosis in exocrine cells or the molecular mechanism underlying this process. We show that Rab27B, a closely related isoform of Rab27A that regulates lysosome-related granule exocytosis in cytotoxic T lymphocytes, is abundantly expressed on amylase-containing secretory granules in rat parotid gland acinar cells. We also identify the putative Rab27B effector protein, Slac2-c (Slp homologue lacking C2 domains-c)/MyRIP, which was originally described as a myosin Va/VIIa and actin binding protein, in rat parotid glands. The results of subcellular fractionation, immunoprecipitation and immunohistochemical studies indicate that the Rab27B-Slac2-c complex is formed on secretory granules in vivo. The introduction of either a specific Rab27 binding domain (i.e. a recombinant Slp homology domain of Slac2-b that specifically binds Rab27A/B but not other Rabs) or functionally blocking antibodies that specifically disrupt Rab27B-Slac2-c complex in vitro strongly inhibited isoproterenol-stimulated amylase release from streptolysin O-permeabilized parotid acinar cells. Our results indicate that the Rab27B-Slac2-c complex is an important constituent of secretory granule exocytosis in parotid acinar cells.

The role of Rab27a in the regulation of melanosome distribution within retinal pigment epithelial cells

Melanosomes within the retinal pigment epithelium (RPE) of mammals have long been thought to exhibit no movement in response to light, unlike fish and amphibian RPE. Here we show that the distribution of melanosomes within the mouse RPE undergoes modest but significant changes with the light cycle. Two hours after light onset, there is a threefold increase in the number of melanosomes in the apical processes that surround adjacent photoreceptors. In skin melanocytes, melanosomes are motile and evenly distributed throughout the cell periphery. This distribution is due to the interaction with the cortical actin cytoskeleton mediated by a tripartite complex of Rab27a, melanophilin, and myosin Va. In ashen (Rab27a null) mice RPE, melanosomes are unable to move beyond the adherens junction axis and do not enter apical processes, suggesting that Rab27a regulates melanosome distribution in the RPE. Unlike skin melanocytes, the effects of Rab27a are mediated through myosin VIIa in the RPE, as evidenced by the similar melanosome distribution phenotype observed in shaker-1 mice, defective in myosin VIIa. Rab27a and myosin VIIa are likely to be required for association with and movement through the apical actin cytoskeleton, which is a prerequisite for entry into the apical processes.

Pigment cells: a model for the study of organelle transport

Eukaryotic organisms rely on intracellular transport to position organelles and other components within their cells. Pigment cells provide an excellent model to study organelle transport as they specialize in the translocation of pigment granules in response to defined chemical signals. Pigment cells of lower vertebrates have traditionally been used as a model for these studies because these cells transport pigment organelles in a highly coordinated fashion, are easily cultured and transfected, are ideal for microsurgery, and are good for biochemical experiments, including in vitro analysis of organelle motility. Many important properties of organelle transport, for example, the requirement of two cytoskeletal filaments (actin and microtubules), the motor proteins involved, and the mechanisms of their regulation and interactions, have been studied using pigment cells of lower vertebrates. Genetic studies of mouse melanocytes allowed the discovery of essential elements involved in organelle transport including the myosin-Va motor and its receptor and adaptor molecules on the organelle surface. Future studies of pigment cells will contribute to our understanding of issues such as the cooperation among multiple motor proteins and the mechanisms of regulation of microtubule motors.

Missense mutations in the globular tail of myosin-Va indilutemice partially impair binding of Slac2-a/melanophilin

The well-known coat-color mutant mouse dilute exhibits a defect in melanosome transport, and although various mutations in the myosin-Va gene, which encodes an actin-based motor protein, have been identified in dilute mice, why missense mutations in the globular tail of myosin-Va, a putative cargo-binding site, cause the dilute phenotype (i.e. lighter coat color) has never been elucidated. In this study we discovered that missense mutations (I1510N, M1513K and D1519G) in the globular tail (GT) of myosin-Va partially impair the binding of Slac2-a/melanophilin, a linker protein between myosin-Va and Rab27A on the melanosome. The myosin-Va-GT-binding site in Slac2-a was mapped to the region (amino acids 147-240) adjacent to the N-terminal Rab27A-binding site, but it is distinct from the myosin-Va-exon-F-binding site (amino acids 320-406). The myosin-Va-GT·Slac2-a interaction was much weaker than the myosin-Va-exon-F·Slac2-a interaction. The missense mutations in the GT found in dilute mice abrogated only the myosin-Va-GT·Slac2-a interaction and had no effect on the myosin-Va-exon-F·Slac2-a interaction. We further showed that expression of green fluorescence protein-tagged Slac2-a lacking the myosin-Va-GT-binding site (ΔGT), but not the wild-type Slac2-a, severely inhibits melanosome transport in melan-a cells, especially at the melanosome transfer step from microtubles to actin filaments (i.e. perinuclear aggregation of melanosomes). On the basis of our findings, we propose that myosin-Va interacts with Slac2-a·Rab27A complex on the melanosome via two distinct domains, both of which are essential for melanosome transport in melanocytes.

Characterization of Melanosomes in Murine Hermansky–Pudlak Syndrome: Mechanisms of Hypopigmentation

The Hermansky-Pudlak syndrome is a genetically heterogeneous autosomal recessive disorder affecting mice and humans, which causes oculocutaneous albinism, prolonged bleeding, and in some cases, pulmonary fibrosis or granulomatous colitis. We previously demonstrated that the gene defects causing murine Hermansky-Pudlak syndrome cause blocks in melanosome biogenesis and/or trafficking in 10 Hermansky-Pudlak syndrome strains. Here, we report an in vivo quantitative analysis on five additional murine models of the Hermansky-Pudlak syndrome. We demonstrate that all strains examined here except for ashen have defects in morphogenesis, the most severely affected is sandy, muted, and buff followed by subtle gray. The ashen strain only has a defect in secretion, as indicated by retention of melanosomes in melanocytes. We document three cellular mechanisms contributing to the hypopigmentation seen in the Hermansky-Pudlak syndrome: (1) exocytosis of immature hypopigmented melanosomes from melanocytes with subsequent keratinocyte uptake; (2) decreased intramelanocyte steady-state numbers of melanosomes available for transfer to keratinocytes; and (3) accumulation of melanosomes within melanocytes due to defective exocytosis, as seen in ashen. We also report that melanosomes in the DBA/2J strain, the parental strain of the Hermansky-Pudlak syndrome strain sandy, are abnormal, indicating that aberrant biogenesis of melanosomes may play a part in the pathogenesis of pigmentary glaucoma observed in these mice.

Rab27A and its effector MyRIP link secretory granules to F-actin and control their motion towards release sites

The GTPase Rab27A interacts with myosin-VIIa and myosin-Va via MyRIP or melanophilin and mediates melanosome binding to actin. Here we show that Rab27A and MyRIP are associated with secretory granules (SGs) in adrenal chromaffin cells and PC12 cells. Overexpression of Rab27A, GTPase-deficient Rab27A-Q78L, or MyRIP reduced secretory responses of PC12 cells. Amperometric recordings of single adrenal chromaffin cells revealed that Rab27A-Q78L and MyRIP reduced the sustained component of release. Moreover, these effects on secretion were partly suppressed by the actin-depolymerizing drug latrunculin but strengthened by jasplakinolide, which stabilizes the actin cortex. Finally, MyRIP and Rab27A-Q78L restricted the motion of SGs in the subplasmalemmal region of PC12 cells, as measured by evanescent-wave fluorescence microscopy. In contrast, the Rab27A-binding domain of MyRIP and a MyRIP construct that interacts with myosin-Va but not with actin increased the mobility of SGs. We propose that Rab27A and MyRIP link SGs to F-actin and control their motion toward release sites through the actin cortex.

Munc13-4 is a GTP-Rab27-binding protein regulating dense core granule secretion in platelets

Platelets store self-agonists such as ADP and serotonin in dense core granules. Although exocytosis of these granules is crucial for hemostasis and thrombosis, the underlying mechanism is not fully understood. Here, we show that incubation of permeabilized platelets with unprenylated active mutant Rab27A-Q78L, wild type Rab27A, and Rab27B inhibited the secretion, whereas inactive mutant Rab27A-T23N and other GTPases had no effects. Furthermore, we affinity-purified a GTP-Rab27A-binding protein in platelets and identified it as Munc13-4, a homologue of Munc13-1 known as a priming factor for neurotransmitter release. Recombinant Munc13-4 directly bound to GTP-Rab27A and -Rab27B in vitro, but not other GTPases, and enhanced secretion in an in vitro assay. The inhibition of secretion by unprenylated Rab27A was rescued by the addition of Munc13-4, suggesting that Munc13-4 mediates the function of GTP-Rab27. Thus, Rab27 regulates the dense core granule secretion in platelets by employing its binding protein, Munc13-4.

A role for Rab27b in NF-E2-dependent pathways of platelet formation

Megakaryocytes release platelets by reorganizing the cytoplasm into proplatelet extensions. Fundamental to this process is the need to coordinate transport of products and organelles in the appropriate abundance to nascent platelets. The importance of the Rab family of small GTPases (guanosine 5'-triphosphatases) in platelet biogenesis is revealed in gunmetal (gm/gm) mice, which show deficient Rab isoprenylation and macrothrombocytopenia with few granules and abnormal megakaryocyte morphology. Although some Rab proteins are implicated in vesicle and organelle transport along microtubules or actin, the role of any Rab protein in platelet biogenesis is unknown. The limited number of Rab proteins with defective membrane association in gm/gm megakaryocytes prominently includes Rab27a and Rab27b. Normal expression of Rab27b is especially increased with terminal megakaryocyte differentiation and dependent on nuclear factor-erythroid 2 (NF-E2), a transcription factor required for thrombopoiesis. Chromatin immunoprecipitation demonstrates recruitment of NF-E2 to the putative Rab27B promoter. Inhibition of endogenous Rab27 function in primary megakaryocytes causes severe quantitative and qualitative defects in proplatelet formation that mimic findings in gm/gm cells. Rab27b localizes to alpha and dense granules in megakaryocytes. These results establish a role for Rab27 in platelet synthesis and suggest that Rab27b in particular may coordinate proplatelet formation with granule transport, possibly by recruiting specific effector pathways.

Sequences Associated With Human Iris Pigmentation

To determine whether and how common polymorphisms are associated with natural distributions of iris colors, we surveyed 851 individuals of mainly European descent at 335 SNP loci in 13 pigmentation genes and 419 other SNPs distributed throughout the genome and known or thought to be informative for certain elements of population structure. We identified numerous SNPs, haplotypes, and diplotypes (diploid pairs of haplotypes) within the OCA2, MYO5A, TYRP1, AIM, DCT, and TYR genes and the CYP1A2-15q22-ter, CYP1B1-2p21, CYP2C8-10q23, CYP2C9-10q24, and MAOA-Xp11.4 regions as significantly associated with iris colors. Half of the associated SNPs were located on chromosome 15, which corresponds with results that others have previously obtained from linkage analysis. We identified 5 additional genes (ASIP, MC1R, POMC, and SILV) and one additional region (GSTT2-22q11.23) with haplotype and/or diplotypes, but not individual SNP alleles associated with iris colors. For most of the genes, multilocus gene-wise genotype sequences were more strongly associated with iris colors than were haplotypes or SNP alleles. Diplotypes for these genes explain 15% of iris color variation. Apart from representing the first comprehensive candidate gene study for variable iris pigmentation and constituting a first step toward developing a classification model for the inference of iris color from DNA, our results suggest that cryptic population structure might serve as a leverage tool for complex trait gene mapping if genomes are screened with the appropriate ancestry informative markers.

Do disorders of movement cause movement disorders and dementia?

Neurons require long-distance microtubule-based transport systems to ferry vital cellular cargoes and signals between cell bodies and axonal or dendritic terminals. Considerable progress has been made on developing a molecular understanding of these processes and how they are integrated into normal neuronal functions. Recent work also suggests that these transport systems may fail early in the pathogenesis of a number of neurodegenerative diseases.

Integration of proteomics and genomics in platelets: a profile of platelet proteins and platelet-specific genes

Platelets, while anucleate, contain RNA, some of which is translated into protein upon activation. Hypothesising that the platelet proteome is reflected in the transcriptome, we identified 82 proteins secreted from activated platelets and compared these, as well as published proteomic data, to the transcriptional profile. We also compared the transcriptome of platelets to other tissues to identify platelet-specific genes and used ontology to determine gene categories over-represented in platelets. RNA was isolated from highly pure platelet preparations for hybridization to Affymetrix oligonucleotide arrays. We identified 2,928 distinct messages as being present in platelets. The platelet transcriptome was compared with the proteome by relating both to UniGene clusters. Platelet proteomic data correlated well with the transcriptome, with 69% of secreted proteins detectable at the mRNA level, and similar concordance was obtained using two published datasets. While many of the most abundant mRNAs are for known platelet proteins, messages were detected for proteins not previously reported in platelets. Some of these may represent residual megakaryocyte messages; however, proteomic analysis confirmed the expression of many previously unreported genes in platelets. Transcripts for well-described platelet proteins are among the most platelet-specific messages. Ontological categories related to signal transduction, receptors, ion channels, and membranes are over-represented in platelets, while categories involved in protein synthesis are depleted. Despite the absence of gene transcription, the platelet proteome is mirrored in the transcriptome. Conversely, transcriptional analysis predicts the presence of novel proteins in the platelet. Transcriptional analysis is relevant to platelet biology, providing insights into platelet function and the mechanisms of platelet disorders.

A general role for Rab27a in secretory cells

Vesicular transport is a complex multistep process regulated by distinct Rab GTPases. Here, we show for the first time that an EGFP-Rab fusion protein is fully functional in a mammalian organism. We constructed a PAC-based transgenic mouse, which expresses EGFP-Rab27a under the control of endogenous Rab27a promoter. The EGFP-Rab27a transgene was fully functional and rescued the two major defects of the ashen Rab27a knockout mouse. We achieved cell-specific expression of EGFP-Rab27a, which faithfully followed the pattern of expression of endogenous Rab27a. We found that Rab27a is expressed in an exceptionally broad range of specialized secretory cells, including exocrine (particularly in mucin- and zymogen-secreting cells), endocrine, ovarian, and hematopoietic cells, most of which undergo regulated exocytosis. We suggest that Rab27a acts in concert with Rab3 proteins in most regulated secretory events. The present strategy represents one way in which the complex pattern of expression and function of proteins involved in specialized cell types may be unraveled.

Rab proteins and endocytic trafficking: potential targets for therapeutic intervention

Rab GTPases serve as master regulators of vesicular membrane transport on both the exo- and endocytic pathways. In their active forms, rab proteins serve in cargo selection and as scaffolds for the sequential assembly of effectors requisite for vesicle budding, cytoskeletal transport, and target membrane fusion. Rab protein function is in turn tightly regulated at the level of protein expression, localization, membrane association, and activation. Alterations in the rab GTPases and associated regulatory proteins or effectors have increasingly been implicated in causing human disease. Some diseases such as those resulting in bleeding and pigmentation disorders (Griscelli syndrome), mental retardation, neuropathy (Charcot-Marie-Tooth), kidney disease (tuberous sclerosis), and blindness (choroideremia) arise from direct loss of function mutations of rab GTPases or associated regulatory molecules. In contrast, in a number of cancers (prostate, liver, breast) as well as vascular, lung, and thyroid diseases, the overexpression of select rab GTPases have been tightly correlated with disease pathogenesis. Unique therapeutic opportunities lie ahead in developing strategies that target rab proteins and modulate the endocytic pathway.

The orphan granzymes of humans and mice

The granzyme/perforin pathway is a central pathway for lymphocyte-mediated killing in both the innate and adaptive immune systems. This pathway is important in a variety of host defenses, including viral clearance and tumor cell killing, and its dysregulation results in several human and rodent diseases. To date, the majority of reports in this field have concentrated on the functions of granzymes A and B. Recent reports, however, suggest that the non-A/non-B 'orphan' granzymes found in both humans and mice are potentially significant. Although the functions of these orphan granzymes have yet to be fully established, initial data suggests their importance in both immune and nonimmune cells.

A defect in a novel ADAMTS family member is the cause of the belted white-spotting mutation

Several features of the pigment defect in belted (bt) mutant mice suggest that it occurs as a result of a defect in melanocyte development that is unique from those described for other classical white-spotting mutations. We report here that bt mice carry mutations in Adamts20, a novel member of the ADAMTS family of secreted metalloproteases. Adamts20 shows a highly dynamic pattern of expression in the developing embryo that generally precedes the appearance of melanoblasts in the same region, and is not expressed in the migrating cells themselves. Adamts20 shows remarkable homology with GON-1, an ADAMTS family protease required for distal tip cell migration in C. elegans. Our results suggest that the role of ADAMTS proteases in the regulation of cell migration has been conserved in mammalian development.

Biochemical and molecular characterization of diseases linked to motor proteins

Recent studies have revealed that kinesin, dynein and myosin each form large superfamilies and participate in many different intracellular transport systems. Importantly, these motor proteins play significant roles in the pathogenesis of a variety of diseases. Studies using knockout mice for kinesin KIF1B have led to the identification of the cause of a human hereditary neuropathy, Charcot-Marie-Tooth disease type 2A. The function of members of the dynein superfamily whose existence has previously only been confirmed through genome databases, has been revealed by studies of immotile cilia syndrome. Unconventional myosins have been shown to function in the inner-ear cells by examination of hereditary human hearing impairment and studies using mouse models. In addition, some diseases are caused by mutations, not in the motor itself, but in the proteins associated with the motor proteins. Here, we discuss the relationship of these motor proteins and how they contribute to disease in molecular terms.

Aberrant lung structure, composition, and function in a murine model of Hermansky-Pudlak syndrome

Hermansky-Pudlak syndrome (HPS) is a genetically heterogeneous inherited disease causing hypopigmentation and prolonged bleeding times. An additional serious clinical problem of HPS is the development of lung pathology, which may lead to severe lung disease and premature death. No cure for the disease exists, and previously, no animal model for the HPS lung abnormalities has been reported. A mouse model of HPS, which is homozygously recessive for both the Hps1 (pale ear) and Hps2 (pearl) genes, exhibits striking abnormalities of lung type II cells. Type II cells and lamellar bodies of this mutant are greatly enlarged, and the lamellar bodies are engorged with surfactant. Mutant lungs accumulate excessive autofluorescent pigment. The air spaces of mutant lungs contain age-related elevations of inflammatory cells and foamy macrophages. In vivo measurement of lung hysteresivity demonstrated aberrant lung function in mutant mice. All these features are similar to the lung pathology described in HPS patients. Morphometry of mutant lungs indicates a significant emphysema. These mutant mice provide a model to further investigate the lung pathology and therapy of HPS. We hypothesize that abnormal type II cell lamellar body structure/function may predict future lung pathology in HPS.

The actin-binding domain of Slac2-a/melanophilin is required for melanosome distribution in melanocytes

Melanosomes containing melanin pigments are transported from the cell body of melanocytes to the tips of their dendrites by a combination of microtubule- and actin-dependent machinery. Three proteins, Rab27A, myosin Va, and Slac2-a/melanophilin (a linker protein between Rab27A and myosin Va), are known to be essential for proper actin-based melanosome transport in melanocytes. Although Slac2-a directly interacts with Rab27A and myosin Va via its N-terminal region (amino acids 1 to 146) and the middle region (amino acids 241 to 405), respectively, the functional importance of the putative actin-binding domain of the Slac2-a C terminus (amino acids 401 to 590) in melanosome transport has never been elucidated. In this study we showed that formation of a tripartite protein complex between Rab27A, Slac2-a, and myosin Va alone is insufficient for peripheral distribution of melanosomes in melanocytes and that the C-terminal actin-binding domain of Slac2-a is also required for proper melanosome transport. When a Slac2-a deletion mutant (DeltaABD) or point mutant (KA) that lacks actin-binding ability was expressed in melanocytes, the Slac2-a mutants induced melanosome accumulation in the perinuclear region, possibly by a dominant negative effect, the same as the Rab27A-binding-defective mutant of Slac2-a or the myosin Va-binding-defective mutant. Our findings indicate that Slac2-a organizes actin-based melanosome transport in cooperation with Rab27A, myosin Va, and actin.

Griscelli syndrome restricted to hypopigmentation results from a melanophilin defect (GS3) or a MYO5A F-exon deletion (GS1)

Griscelli syndrome (GS) is a rare autosomal recessive disorder that associates hypopigmentation, characterized by a silver-gray sheen of the hair and the presence of large clusters of pigment in the hair shaft, and the occurrence of either a primary neurological impairment or a severe immune disorder. Two different genetic forms, GS1 and GS2, respectively, account for the mutually exclusive neurological and immunological phenotypes. Mutations in the gene encoding the molecular motor protein Myosin Va (MyoVa) cause GS1 and the dilute mutant in mice, whereas mutations in the gene encoding the small GTPase Rab27a are responsible for GS2 and the ashen mouse model. We herein present genetic and functional evidence that a third form of GS (GS3), whose expression is restricted to the characteristic hypopigmentation of GS, results from mutation in the gene that encodes melanophilin (Mlph), the ortholog of the gene mutated in leaden mice. We also show that an identical phenotype can result from the deletion of the MYO5A F-exon, an exon with a tissue-restricted expression pattern. This spectrum of GS conditions pinpoints the distinct molecular pathways used by melanocytes, neurons, and immune cells in secretory granule exocytosis, which in part remain to be unraveled.

Cappuccino, a mouse model of Hermansky-Pudlak syndrome, encodes a novel protein that is part of the pallidin-muted complex (BLOC-1)

Hermansky-Pudlak syndrome (HPS) is a disorder of organelle biogenesis affecting 3 related organelles-melanosomes, platelet dense bodies, and lysosomes. Four genes causing HPS in humans (HPS1-HPS4) are known, and at least 15 nonallelic mutations cause HPS in the mouse. Where their functions are known, the HPS-associated proteins are involved in some aspect of intracellular vesicular trafficking, that is, protein sorting and vesicle docking and fusion. Biochemical and genetic evidence indicates that the HPS-associated genes encode components of at least 3 distinct protein complexes: the adaptor complex AP-3; the HPS1/HPS4 complex; and BLOC-1 (biogenesis of lysosome-related organelles complex-1), consisting of the proteins encoded at 2 mouse HPS loci, pallid (pa) and muted (mu), and at least 3 other unidentified proteins. Here, we report the cloning of the mouse HPS mutation cappuccino (cno). We show that the wild-type cno gene encodes a novel, ubiquitously expressed cytoplasmic protein that coassembles with pallidin and the muted protein in the BLOC-1 complex. Further, we identify a frameshift mutation in mutant cno/cno mice. The C-terminal 81 amino acids are replaced with 72 different amino acids in the mutant CNO protein, and its ability to interact in BLOC-1 is abolished. We performed mutation screening of patients with HPS and failed to identify any CNO defects. Notably, although defects in components of the HPS1/HPS4 and the AP-3 complexes are associated with HPS in humans, no defects in the known components of BLOC-1 have been identified in 142 patients with HPS screened to date, suggesting that BLOC-1 function may be critical in humans.

The Hermansky-Pudlak syndrome 1 (HPS1) and HPS4 proteins are components of two complexes, BLOC-3 and BLOC-4, involved in the biogenesis of lysosome-related organelles

Hermansky-Pudlak syndrome (HPS) is a genetic disease of lysosome, melanosome, and granule biogenesis. Mutations of six different loci have been associated with HPS in humans, the most frequent of which are mutations of the HPS1 and HPS4 genes. Here, we show that the HPS1 and HPS4 proteins are components of two novel protein complexes involved in biogenesis of melanosome and lysosome-related organelles: biogenesis of lysosome-related organelles complex-(BLOC) 3 and BLOC-4. The phenotypes of Hps1-mutant (pale-ear; ep) and Hps4-mutant (light-ear; le) mice and humans are very similar, and cells from ep and le mice exhibit similar abnormalities of melanosome morphology. HPS1 protein is absent from ep-mutant cells, and HPS4 from le-mutant cells, but le-mutant cells also lack HPS1 protein. HPS4 protein seems to be necessary for stabilization of HPS1, and the HPS1 and HPS4 proteins co-immunoprecipitate, indicating that they are in a complex. HPS1 and HPS4 do not interact directly in a yeast two-hybrid system, although HPS4 interacts with itself. In a partially purified vesicular/organellar fraction, HPS1 and HPS4 are both components of a complex with a molecular mass of approximately 500 kDa, termed BLOC-3. Within BLOC-3, HPS1 and HPS4 are components of a discrete approximately 200-kDa module termed BLOC-4. In the cytosol, HPS1 (but not HPS4) is part of yet another complex, termed BLOC-5. We propose that the BLOC-3 and BLOC-4 HPS1.HPS4 complexes play a central role in trafficking cargo proteins to newly formed cytoplasmic organelles.

Delivering the kiss of death

Cytotoxic T lymphocytes and natural killer cells kill their targets by secreting specialized granules that contain potent cytotoxic molecules. Through the study of rare immunodeficiency diseases in which this granule pathway of killing is impaired, proteins such as Rab27a have been identified as components of the secretory machinery of these killer cells. Recent evidence suggests that the destruction of activated lymphocytes through granule-mediated killing may be an important mechanism of immunological homeostasis. Although the process by which this occurs is not yet known, it is possible that events taking place at the immunological synapse may render the killer cell susceptible to fratricidal attack by other killer cells.

Slp4-a/granuphilin-a inhibits dense-core vesicle exocytosis through interaction with the GDP-bound form of Rab27A in PC12 cells

Slp4-a (synaptotagmin-like protein 4-a)/granuphilin-a is specifically localized on dense-core vesicles in PC12 cells and negatively controls dense-core vesicle exocytosis through specific interaction with Rab27A via the N-terminal Slp homology domain (SHD) (Fukuda, M., Kanno, E., Saegusa, C., Ogata, Y., and Kuroda, T. S. (2002) J. Biol. Chem. 277, 39673-39678). However, the mechanism of the inhibition by Slp4-a has never been elucidated at the molecular level and is still a matter of controversy. In this study, I discovered an unexpected biochemical property of Slp4-a, that Slp4-a, but not other Rab27 effectors reported thus far, is capable of interacting with both Rab27A(T23N), a dominant negative form that mimics the GDP-bound form, and Rab27A(Q78L), a dominant active form that mimics the GTP-bound form, whereas Slp4-a specifically recognizes the GTP-bound form of Rab3A and Rab8A and does not recognize their GDP-bound form. I show by deletion and mutation analyses that the TGDWFY sequence in SHD2 is essential for Rab27A(T23N) binding, whereas SHD1 is involved in Rab27A(Q78L) binding. I further show by immunoprecipitation and cotransfection assays that Munc18-1, but not syntaxin IA, directly interacts with the C-terminal domain of Slp4-a in a Rab27A-independent manner. Expression of Slp4-a mutants that lack Rab27A(T23N) binding activity (i.e. specific binding to Rab27A(Q78L)) completely reverses the inhibitory effect of the wild-type Slp4-a on high KCl-dependent neuropeptide Y secretion in PC12 cells. The results strongly indicate that interaction of Slp4-a with the GDP-bound form of Rab27A, not with syntaxin IA or Munc18-1, is the primary reason that Slp4-a expression inhibits dense core vesicle exocytosis in PC12 cells.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Biochemical and functional characterization of Rab27a mutations occurring in Griscelli syndrome patients

Rab27a is a member of the Rab family of small GTPase proteins, and thus far is the first member to be associated with a human disease (ie, the Griscelli syndrome type 2). Mutations in the Rab27a gene cause pigment as well as cytotoxic granule transport defects, accounting for the partial albinism and severe immune disorder characteristics of this syndrome. So far, 3 Rab27a missense mutations have been identified. They open a unique opportunity to designate critical structural and functional residues of Rab proteins. We show here that the introduction of a proline residue in the alpha 4 (Ala152Pro) or beta 5 (Leu130Pro) loop, observed in 2 of these spontaneous mutants, dramatically affects both guanosine triphosphate (GTP) and guanosine diphosphate (GDP) nucleotide-binding activity of Rab27a, probably by disrupting protein folding. The third mutant, Trp73Gly, is located within an invariant hydrophobic triad at the switch interface, and was previously shown in active Rab3A to mediate rabphilin3A effector interaction. Trp73Gly is shown to display the same nucleotide-binding and GTPase characteristics as the constitutively active mutant Gln78Leu. However, in contrast to Gln78Leu, Trp73Gly mutant construct neither interacts with the Rab27a effector melanophilin nor modifies melanosome distribution and cytotoxic granule exocytosis. Substitutions introduced at the 73 position, including the leucine residue present in Ras, did not restore Rab27a protein functions. Taken together, our results characterize new critical residues of Rab proteins, and identify the Trp73 residue of Rab27a as a key position for interaction with the specific effectors of Rab27a, both in melanocytes and cytotoxic cells.