Involvement of vps33a in the fusion of uroplakin-degrading multivesicular bodies with lysosomes.

The apical surface of the terminally differentiated mouse bladder urothelium is largely covered by urothelial plaques, consisting of hexagonally packed 16-nm uroplakin particles. These plaques are delivered to the cell surface by fusiform vesicles (FVs) that are the most abundant cytoplasmic organelles. We have analyzed the functional involvement of several proteins in the apical delivery and endocytic degradation of uroplakin proteins. Although FVs have an acidified lumen and Rab27b, which localizes to these organelles, is known to be involved in the targeting of lysosome-related organelles (LROs), FVs are CD63 negative and are therefore not typical LROs. Vps33a is a Sec1-related protein that plays a role in vesicular transport to the lysosomal compartment. A point mutation in mouse Vps33a (Buff mouse) causes albinism and bleeding (Hermansky-Pudlak syndrome) because of abnormalities in the trafficking of melanosomes and platelets. These Buff mice showed a novel phenotype observed in urothelial umbrella cells, where the uroplakin-delivering FVs were almost completely replaced by Rab27b-negative multivesicular bodies (MVBs) involved in uroplakin degradation. MVB accumulation leads to an increase in the amounts of uroplakins, Lysosomal-associated membrane protein (LAMP)-1/2, and the activities of beta-hexosaminidase and beta-glucocerebrosidase. These results suggest that FVs can be regarded as specialized secretory granules that deliver crystalline arrays of uroplakins to the cell surface, and that the Vps33a mutation interferes with the fusion of MVBs with mature lysosomes thus blocking uroplakin degradation.

The Vps33a gene regulates behavior and cerebellar Purkinje cell number.

A mutation in the Vps33a gene causes Hermansky-Pudlak Syndrome (HPS)-like-symptoms in the buff (bf) mouse mutant. The encoded product, Vps33a, is a member of the Sec1 and Class C multi-protein complex that regulates vesicle trafficking to specialized lysosome-related organelles. As Sec1 signaling pathways have been implicated in pre-synaptic function, we examined brain size, cerebellar cell number and the behavioral phenotype of bf mutants. Standardized behavioral tests (SHIRPA protocols) demonstrated significant motor deficits (e.g., grip strength, righting reflex and touch escape) in bf mutants, worsening with age. Histological examination of brain revealed significant Purkinje cell loss that was confirmed with staining for calbindin, a calcium binding protein enriched in Purkinje cells. This pathologic finding was progressive, as older bf mutants (13-14 months) showed a greater attrition of neurons, with their cerebella appearing to be particularly reduced (approximately 30%) in size relative to those of age-matched-control cohorts. These studies suggest that loss of Purkinje neurons is the most obvious neurological atrophy in the bf mutant, a structural change that generates motor coordination deficits and impaired postural phenotypes. It is conceivable therefore that death of cerebellar cells may also be a clinical feature of HPS patients, a pathological event which has not been reported in the literature. In general, the bf mutant may be a potentially new and useful model for understanding Purkinje cell development and function.

Murine leukemia virus spreading in mice impaired in the biogenesis of secretory lysosomes and Ca2+-regulated exocytosis.

BACKGROUND: Retroviruses have been observed to bud intracellularly into multivesicular bodies (MVB), in addition to the plasma membrane. Release from MVB is thought to occur by Ca(2+)-regulated fusion with the plasma membrane. PRINCIPAL FINDINGS: To address the role of the MVB pathway in replication of the murine leukemia virus (MLV) we took advantage of mouse models for the Hermansky-Pudlak syndrome (HPS) and Griscelli syndrome. In humans, these disorders are characterized by hypopigmentation and immunological alterations that are caused by defects in the biogenesis and trafficking of MVBs and other lysosome related organelles. Neonatal mice for these disease models lacking functional AP-3, Rab27A and BLOC factors were infected with Moloney MLV and the spread of virus into bone marrow, spleen and thymus was monitored. We found a moderate reduction in MLV infection levels in most mutant mice, which differed by less than two-fold compared to wild-type mice. In vitro, MLV release form bone-marrow derived macrophages was slightly enhanced. Finally, we found no evidence for a Ca(2+)-regulated release pathway in vitro. Furthermore, MLV replication was only moderately affected in mice lacking Synaptotagmin VII, a Ca(2+)-sensor regulating lysosome fusion with the plasma membrane. CONCLUSIONS: Given that MLV spreading in mice depends on multiple rounds of replication even moderate reduction of virus release at the cellular level would accumulate and lead to a significant effect over time. Thus our in vivo and in vitro data collectively argue against an essential role for a MVB- and secretory lysosome-mediated pathway in the egress of MLV.

The Slc35d3 gene, encoding an orphan nucleotide sugar transporter, regulates platelet-dense granules.

Platelet dense granules are lysosome-related organelles which contain high concentrations of several biologically important low-molecular-weight molecules. These include calcium, serotonin, adenine nucleotides, pyrophosphate, and polyphosphate, which are necessary for normal blood hemostasis. The synthesis of dense granules and other lysosome-related organelles is defective in inherited diseases such as Hermansky-Pudlak syndrome (HPS) and Chediak-Higashi syndrome (CHS). HPS and CHS mutations in 8 human and at least 16 murine genes have been identified. Previous studies produced contradictory findings for the function of the murine ashen (Rab27a) gene in platelet-dense granules. We have used a positional cloning approach with one line of ashen mutants to establish that a new mutation in a second gene, Slc35d3, on mouse chromosome 10 is the basis of this discrepancy. The platelet-dense granule defect is rescued in BAC transgenic mice containing the normal Slc35d3 gene. Thus, Slc35d3, an orphan member of a nucleotide sugar transporter family, specifically regulates the contents of platelet-dense granules. Unlike HPS or CHS genes, it has no apparent effect on other lysosome-related organelles such as melanosomes or lysosomes. The ash-Roswell mouse mutant is an appropriate model for human congenital-isolated delta-storage pool deficiency.

Interaction of Hermansky-Pudlak Syndrome genes in the regulation of lysosome-related organelles.

Hermansky-Pudlak Syndrome (HPS) is a genetically heterogeneous disease caused by abnormalities in the synthesis and/or trafficking of lysosome-related organelles (LROs) including melanosomes, lamellar bodies of lung type II cells and platelet dense granules. At least 15 genes cause HPS in mice, with a significant number specifying novel subunits of protein complexes termed BLOCs (Biogenesis of Lysosome-related Organelles Complexes). To ascertain whether BLOC complexes functionally interact in vivo, mutant mice doubly or triply deficient in protein subunits of the various BLOC complexes and/or the AP-3 adaptor complex were constructed and tested for viability and for abnormalities of melanosomes, lung lamellar bodies and lysosomes. All mutants, including those deficient in all three BLOC complexes, were viable though the breeding efficiencies of multiple mutants involving AP-3 were severely compromised. Interactions of BLOC protein complexes with each other and with AP-3 to affect most LROs were apparent. However, these interactions were tissue and organelle dependent. These studies document novel biological interactions of BLOC and AP-3 complexes in the biosynthesis of LROs and assess the role(s) of HPS protein complexes in general health and physiology in mammals. Double and triple mutant HPS mice provide unique and practical experimental advantages in the study of LROs.

Slc7a11 gene controls production of pheomelanin pigment and proliferation of cultured cells.

In mammals, >100 genes regulate pigmentation by means of a wide variety of developmental, cellular, and enzymatic mechanisms. Nevertheless, genes that directly regulate pheomelanin production have not been described. Here, we demonstrate that the subtle gray (sut) mouse pigmentation mutant arose by means of a mutation in the Slc7a11 gene, encoding the plasma membrane cystine/glutamate exchanger xCT [Kanai, Y. & Endou, H. (2001) Curr. Drug Metab. 2, 339-354]. A resulting low rate of extracellular cystine transport into sut melanocytes reduces pheomelanin production. We show that Slc7a11 is a major genetic regulator of pheomelanin pigment in hair and melanocytes, with minimal or no effects on eumelanin. Furthermore, transport of cystine by xCT is critical for normal proliferation, glutathione production, and protection from oxidative stress in cultured cells. Thus, we have found that the Slc7a11 gene controls the production of pheomelanin pigment directly. Cells from sut mice provide a model for oxidative stress-related diseases and their therapies.

Defective surfactant secretion in a mouse model of Hermansky-Pudlak syndrome.

Hermansky-Pudlak syndrome (HPS) in humans represents a family of disorders of lysosome-related organelle biogenesis associated with severe, progressive pulmonary disease. Human case reports and a mouse model of HPS, the pale ear/pearl mouse (ep/pe), exhibit giant lamellar bodies (GLB) in type II alveolar epithelial cells. We examined surfactant proteins and phospholipid from ep/pe mice to elucidate the process of GLB formation. The 2.8-fold enrichment of tissue phospholipids in ep/pe mice resulted from accumulation from birth through adulthood. Tissue surfactant protein (SP)-B and -C were increased in adult ep/pe mice compared with wild-type mice (WT), whereas SP-A and -D were not different. Large aggregate surfactant (LA) from adult ep/pe mice had decreased phospholipid, SP-B, and SP-C, with no differences in SP-A and -D compared with WT. Although LA from ep/pe animals exhibited an increased total protein-to-total phospholipid ratio compared with WT, surface tension was not compromised. Phospholipid secretion from isolated type II cells showed that basal and stimulated secretion from ep/pe cells were approximately 50% of WT cells. Together, our data indicate that GLB formation is not associated with abnormal trafficking or recycling of surfactant material. Instead, impaired secretion is an important component of GLB formation in ep/pe mice.

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.

The Hermansky-Pudlak syndrome 3 (cocoa) protein is a component of the biogenesis of lysosome-related organelles complex-2 (BLOC-2).

Hermansky-Pudlak syndrome (HPS) is a genetically heterogeneous inherited disease affecting vesicle trafficking among lysosome-related organelles. The Hps3, Hps5, and Hps6 genes are mutated in the cocoa, ruby-eye-2, and ruby-eye mouse pigment mutants, respectively, and their human orthologs are mutated in HPS3, HPS5, and HPS6 patients. These three genes encode novel proteins of unknown function. The phenotypes of Hps5/Hps5,Hps6/Hps6 and Hps3/Hps3,Hps6/Hps6 double mutant mice mimic, in coat and eye colors, in melanosome ultrastructure, and in levels of platelet dense granule serotonin, the corresponding phenotypes of single mutants. These facts suggest that the proteins encoded by these genes act within the same pathway or protein complex in vivo to regulate vesicle trafficking. Further, the Hps5 protein is destabilized within tissues of Hps3 and Hps6 mutants, as is the Hps6 protein within tissues of Hps3 and Hps5 mutants. Also, proteins encoded by these genes co-immunoprecipitate and occur in a complex of 350 kDa as determined by sucrose gradient and gel filtration analyses. Together, these results indicate that the Hps3, Hps5, and Hps6 proteins regulate vesicle trafficking to lysosome-related organelles at the physiological level as components of the BLOC-2 (biogenesis of lysosome-related organelles complex-2) protein complex and suggest that the pathogenesis and future therapies of HPS3, HPS5, and HPS6 patients are likely to be similar. Interaction of the Hps5 and Hps6 proteins within BLOC-2 is abolished by the three-amino acid deletion in the Hps6(ru) mutant allele, indicating that these three amino acids are important for normal BLOC-2 complex formation.

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.

Hermansky-Pudlak syndrome type 7 (HPS-7) results from mutant dysbindin, a member of the biogenesis of lysosome-related organelles complex 1 (BLOC-1).

Hermansky-Pudlak syndrome (HPS; MIM 203300) is a genetically heterogeneous disorder characterized by oculocutaneous albinism, prolonged bleeding and pulmonary fibrosis due to abnormal vesicle trafficking to lysosomes and related organelles, such as melanosomes and platelet dense granules. In mice, at least 16 loci are associated with HPS, including sandy (sdy; ref. 7). Here we show that the sdy mutant mouse expresses no dysbindin protein owing to a deletion in the gene Dtnbp1 (encoding dysbindin) and that mutation of the human ortholog DTNBP1 causes a novel form of HPS called HPS-7. Dysbindin is a ubiquitously expressed protein that binds to alpha- and beta-dystrobrevins, components of the dystrophin-associated protein complex (DPC) in both muscle and nonmuscle cells. We also show that dysbindin is a component of the biogenesis of lysosome-related organelles complex 1 (BLOC-1; refs. 9-11), which regulates trafficking to lysosome-related organelles and includes the proteins pallidin, muted and cappuccino, which are associated with HPS in mice. These findings show that BLOC-1 is important in producing the HPS phenotype in humans, indicate that dysbindin has a role in the biogenesis of lysosome-related organelles and identify unexpected interactions between components of DPC and BLOC-1.

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 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.

The mouse organellar biogenesis mutant buff results from a mutation in Vps33a, a homologue of yeast vps33 and Drosophila carnation.

In the mouse, more than 16 loci are associated with mutant phenotypes that include defective pigmentation, aberrant targeting of lysosomal enzymes, prolonged bleeding, and immunodeficiency, the result of defective biogenesis of cytoplasmic organelles: melanosomes, lysosomes, and various storage granules. Many of these mouse mutants are homologous to the human Hermansky-Pudlak syndrome (HPS), Chediak-Higashi syndrome, and Griscelli syndrome. We have mapped and positionally cloned one of these mouse loci, buff (bf), which has a mutant phenotype similar to that of human HPS. Mouse bf results from a mutation in Vps33a and thus is homologous to the yeast vacuolar protein-sorting mutant vps33 and Drosophila carnation (car). This is the first found defect of the class C vacuole/prevacuole-associated target soluble N-ethylmaleimide-sensitive factor attachment protein receptor (t-SNARE) complex in mammals and the first mammalian mutant found that is directly homologous to a vps mutation of yeast. VPS33A thus is a good candidate gene for a previously uncharacterized form of human HPS.

Ru2 and Ru encode mouse orthologs of the genes mutated in human Hermansky-Pudlak syndrome types 5 and 6.

Hermansky-Pudlak syndrome (HPS) is a genetically heterogeneous disease involving abnormalities of melanosomes, platelet dense granules and lysosomes. Here we have used positional candidate and transgenic rescue approaches to identify the genes mutated in ruby-eye 2 and ruby-eye mice (ru2 and ru, respectively), two 'mimic' mouse models of HPS. We also show that these genes are orthologs of the genes mutated in individuals with HPS types 5 and 6, respectively, and that their protein products directly interact. Both genes are previously unknown and are found only in higher eukaryotes, and together represent a new class of genes that have evolved in higher organisms to govern the synthesis of highly specialized lysosome-related organelles.

Melanosome morphologies in murine models of hermansky-pudlak syndrome reflect blocks in organelle development.

Hermansky-Pudlak syndrome is an autosomal recessive disease characterized by pigment dilution and prolonged bleeding time. At least 15 mutant mouse strains have been classified as models of Hermansky-Pudlak syndrome. Some of the genes are implicated in intracellular vesicle trafficking: budding, targeting, and secretion. Many of the Hermansky-Pudlak syndrome genes remain uncharacterized and their functions are unknown. Clues to the functions of these genes can be found by analyzing the physiologic and cellular phenotypes. Here we have examined the morphology of the melanosomes in the skin of 10 of the mutant mouse Hermansky-Pudlak syndrome strains by transmission electron microscopy. We demonstrate that the morphologies reflect inhibition of organelle maturation or transfer. The Hermansky-Pudlak syndrome strains are classified into morphologic groups characterized by the step at which melanosome biogenesis or transfer to keratinocytes is inhibited, with the cappuccino strain observed to be blocked at the earliest step and gunmetal blocked at the latest step. We show that all Hermansky-Pudlak syndrome mutant strains except gunmetal have an increase in unpigmented or hypopigmented immature melanosomal forms, leading to the hypopigmented coat colors seen in these strains. In contrast, the hypopigmentation seen in the gunmetal strain is due to the retention of melanosomes in melanocytes, and inefficient transfer into keratinocytes.

The regulation of platelet-dense granules by Rab27a in the ashen mouse, a model of Hermansky-Pudlak and Griscelli syndromes, is granule-specific and dependent on genetic background.

The ashen (ash) mouse, a model for Hermansky-Pudlak syndrome (HPS) and for a subset of patients with Griscelli syndrome, presents with hypopigmentation, prolonged bleeding times, and platelet storage pool deficiency due to a mutation which abrogates expression of the Rab27a protein. Platelets of mice with the ashen mutation on the C3H/HeSnJ inbred strain background have greatly reduced amounts of dense granule components such as serotonin and adenine nucleotides though near-normal numbers of dense granules as enumerated by the dense granule-specific fluorescent dye mepacrine. Thus, essentially normal numbers of platelet dense granules are produced but the granule interiors are abnormal. Collagen-mediated aggregation of mutant platelets is significantly depressed. No abnormalities in the concentrations or secretory rates of 2 other major platelet granules, lysosomes and alpha granules, were apparent. Similarly, no platelet ultrastructural alterations other than those involving dense granules were detected. Therefore, Rab27a regulates the synthesis and secretion of only one major platelet organelle, the dense granule. There were likewise no mutant effects on levels or secretion of lysosomal enzymes of several other tissues. Together with other recent analyses of the ashen mouse, these results suggest a close relationship between platelet dense granules, melanosomes of melanocytes and secretory lysosomes of cytotoxic T lymphocytes, all mediated by Rab27a. Surprisingly, the effects of the ashen mutation on platelet-dense granule components, platelet aggregation, and bleeding times were highly dependent on genetic background. This suggests that bleeding tendencies may likewise vary among patients with Griscelli syndrome and HPS with Rab27a mutations.

Pulmonary pathologies in pallid mice result from nonhematopoietic defects.

Several single gene pigment mutants of inbred C57BL/6J mice display a triad of subcellular granule-associated defects: oculocutaneous pigment dilution, prolonged bleeding due to defects in platelet dense granules, and abnormal lysosomes. These features also characterize Hermansky-Pudlak Syndrome (HPS), making these mice relevant animal models for HPS. Mice of one mutant strain, pallid, in addition to the hallmark triad of signs, also exhibit age-dependent lung pathology. Respiratory system mechanics showed that the age-dependent histopathology of pallid mice was accompanied by a decrease in lung reactance. Furthermore, the possibility that pallid pulmonary pathology may result from persistent inflammation due to microhemorrhage owing to the platelet defect was examined. Hematopoietic reconstitution of pallid mice with marrow from normal C57BL/6J donors did not prevent the development of the pulmonary histopathology or respiratory system mechanics characteristic of the pallid genotype. Similarly, wild-type mice 12 months after engraftment with pallid marrow did not develop pallid-like pulmonary histopathology or respiratory system mechanics. Thus, pallid-associated pulmonary functional and structural pathologies are not linked to the marrow (bleeding) genotype, but instead are the result of an age-dependent process resulting from a defect(s) in one or more nonhematopoietic cell types.

Cell-specific abnormal prenylation of Rab proteins in platelets and melanocytes of the gunmetal mouse.

The mutant gunmetal mouse exhibits reduced rates of platelet synthesis, abnormalities of platelet alpha and dense granules and hypopigmentation. Several of these features resemble those of human alpha/delta platelet storage pool disease, grey platelet syndrome and Hermansky-Pudlak syndrome. Gunmetal mice have reduced levels of Rab geranylgeranyltransferase (RabGGTase), which adds lipophilic prenyl groups to the carboxyl terminus of Rab proteins. The degree of prenylation and the subcellular distribution of several Rab proteins were evaluated in mutant platelets, melanocytes and other tissues. Significant deficits in prenylation and membrane binding of most Rabs were observed in platelets and melanocytes. In contrast, minimal alterations in Rab prenylation were apparent in several other gunmetal tissues despite the fact that RabGGTase activity was equally diminished in these tissues. The mutant tissue-specific effects are probably due to increased concentrations of Rab proteins in platelets and melanocytes. These experiments show that Rab proteins are differentially sensitive to levels of RabGGTase activity and that normal platelet synthesis, platelet organelle function and normal pigmentation are highly sensitive to the degree of prenylation and membrane association of Rab proteins. Further, the tissue-specific effects of the gunmetal mutation suggest that RabGGTase is a potential target for therapy of thrombocytosis.

The gene for the muted (mu) mouse, a model for Hermansky-Pudlak syndrome, defines a novel protein which regulates vesicle trafficking.

The muted (mu) mouse is a model for Hermansky-Pudlak Syndrome (HPS), an inherited disorder of humans causing hypopigmentation, hemorrhaging and early death due to lung abnormalities. The mu gene regulates the synthesis of specialized mammalian organelles such as melanosomes, platelet dense granules and lysosomes. Further, balance defects indicate that it controls the synthesis of otoliths of the inner ear. The mu gene has been identified by a positional/candidate approach involving large mouse interspecific backcrosses. It encodes a novel ubiquitously expressed transcript, specifying a predicted 185 amino acid protein, whose expression is abrogated in the mu allele which contains an insertion of an early transposon (ETn) retrotransposon. Expression is likewise expected to be lost in the mu( J) allele which contains a deletion of a single base pair within the coding region. The presence of structurally aberrant melanosomes within the eyes of mutant mice together with localization of the muted protein within vesicles in both the cell body and dendrites of transfected melan-a melanocytes emphasizes the role of the mu gene in vesicle trafficking. The mu gene is present only in mice and humans among analyzed genomes. As is true for several other recently identified mouse HPS genes, the mu gene is absent in lower eukaryotes. Therefore, the mu gene is a member of the novel gene set that has evolved in higher eukaryotes to regulate the synthesis/function of highly specialized subcellular organelles such as melanosomes and platelet dense granules.