Hermansky-Pudlak syndrome and related disorders of organelle formation

SNAREing immunity: the role of SNAREs in the immune system

The trafficking of molecules and membranes within cells is a prerequisite for all aspects of cellular immune functions, including the delivery and recycling of cell-surface proteins, secretion of immune mediators, ingestion of pathogens and activation of lymphocytes. SNARE (soluble-N-ethylmaleimide-sensitive-factor accessory-protein receptor)-family members mediate membrane fusion during all steps of trafficking, and function in almost all aspects of innate and adaptive immune responses. Here, we provide an overview of the roles of SNAREs in immune cells, offering insight into one level at which precision and tight regulation are instilled on immune responses.

Physiological Roles of Clathrin Adaptor AP Complexes: Lessons from Mutant Animals

Clathrin-associated adaptor protein (AP) complexes play a key role in the transport of proteins, by regulating the formation of transport vesicles as well as cargo selection, between organelles of the post-Golgi network, namely, the trans-Golgi network (TGN), endosomes, lysosomes and the plasma membrane. Evidence has been accumulating for the physiological importance of AP complexes. Deficiency in AP-1A or AP-2 results in embryonic lethality in mice, indicating that these AP complexes are essential for normal development of embryos in mammals. In contrast, mutations in the genes encoding subunits of AP-3A cause an autosomal recessive disorder, Hermansky-Pudlak syndrome in human and its disease models in mice. Knockout mice for the neuron-specific AP-3B suffer from epileptic seizure. Further studies on the physiological and pathological aspects of AP complexes will not only be beneficial for better understanding of developmental biology and medical sciences, but also deepen our insight into the molecular mechanisms of vesicular traffic.

Mixing model systems: using zebrafish and mouse inner ear mutants and other organ systems to unravel the mystery of otoconial development

Human vestibular dysfunction is an increasing clinical problem. Degeneration or displacement of otoconia is a significant etiology of age-related balance disorders and Benign Positional Vertigo (BPV). In addition, commonly used antibiotics, such as aminoglycoside antibiotics, can lead to disruption of otoconial structure and function. Despite such clinical significance, relatively little information has been compiled about the development and maintenance of otoconia in humans. Recent studies in model organisms and other mammalian organ systems have revealed some of the proteins and processes required for the normal biomineralization of otoconia and otoliths in the inner ear of vertebrates. Orchestration of extracellular biomineralization requires bringing together ionic and proteinaceous components in time and space. Coordination of these events requires the normal formation of the otocyst and sensory maculae, specific secretion and localization of extracellular matrix proteins, as well as tight regulation of the endolymph ionic environment. Disruption of any of these processes can lead to the formation of abnormally shaped, or ectopic, otoconia, or otoconial agenesis. We propose that normal generation of otoconia requires a complex temporal and spatial control of developmental and biochemical events. In this review, we suggest a new hypothetical model for normal otoconial and otolith formation based on matrix vesicle mineralization in bone which we believe to be supported by information from existing mutants, morphants, and biochemical studies.

Fas-ligand is stored in secretory lysosomes of ocular barrier epithelia and released with microvesicles

Previously we described the release of hr44 from the ciliary epithelium to coincide with the loss of the late endosomal/lysosomal marker protein CD63 in mildly inflamed rat eyes. We showed that both proteins are released with microvesicles into the supernatant of cultured retinal pigmented epithelial cells (ARPE-19). Here we wish to determine whether there is a concomitant loss of fas-ligand (FasL) in vivo and whether ocular epithelial cells have secretory lysosomes similar to T cells, from where FasL and hr44 could derive. FasL plays an important role in immunity, immune cell homeostasis and in the maintenance of immune privilege in the eye. However the mode of release of FasL from ocular epithelial cells or its activity in the eye is not fully understood. In normal rat eyes, FasL was detected in the epithelia of the iris and ciliary body and in the anterior region of the retinal pigmented epithelium. FasL is expressed constitutively and is associated with vesicular structures in the normal ciliary epithelium but is not detectable in the ciliary epithelium of inflamed eyes. In contrast, the posterior RPE, which under normal conditions is negative for FasL and hr44 showed strong staining for both molecules in areas adjacent to sub-retinal inflammatory infiltrates. Immunofluorescence and Western blot analysis indicated that cultured ARPE-19 cells express both the soluble and membrane form of FasL. The intracellular concentration of FasL was significantly increased in cells grown in presence of interferon (INF)-gamma. The microvesicles released by cultured ARPE-19 cells and previously shown to be positive for hr44 and CD63 are also positive for membrane FasL. Expression of a recombinant fluorescent construct of FasL together with immuno-staining for CD63 demonstrated that FasL localises to the endocytic compartment of ARPE-19 cells and of melanoma cells (positive control). In cells with lysosomes devoid of specialised secretory functions (e g. HeLa cells) recombinant FasL localised to the cell membrane, demonstrating that RPE cells have secretory lysosomes. We suggest that ocular epithelial cells release soluble FasL and the membrane form of FasL with vesicles. Both forms may contribute in different ways to the effectiveness of the ocular immune response and immune privilege.

Intestinal disease in Hermansky-Pudlak syndrome: occurrence of colitis and relation to genotype

BACKGROUND & AIMS

Hermansky-Pudlak syndrome (HPS), a rare autosomal recessive disorder characterized by oculocutaneous albinism and platelet dysfunction, results from mutations in 1 of at least 7 different genes. Some patients develop a fatal pulmonary fibrosis and others a disabling colitis. This study aimed to document the occurrence of colitis among HPS patients, characterize gastrointestinal tract involvement in HPS, and analyze the distribution of colitis among HPS genotypes.

METHODS

Of the 122 HPS patients followed at the National Institutes of Health Clinical Center between 1993 and 2005, 24 were evaluated by endoscopy for gastrointestinal complaints. The histology of gastrointestinal biopsies was retrospectively examined to assess for inflammatory changes, granulomata, and pigmented macrophages. These data were compared with symptoms and HPS genetic subtypes.

RESULTS

At colonoscopy, 7 of 23 patients (30%) had endoscopic mucosal abnormalities, including nodularity, erythema, petechiae, or erosions. Six of these 7 patients (86%) had findings of colitis on biopsy. Of the 16 patients with normal-appearing colonic mucosa, 2 patients (12%) had colitis on biopsy. Pigmented macrophages were also observed in the colonic lamina propria in 16 of the 23 patients (70%). Of the 8 patients with confirmed colitis, 7 had the HPS-1 subtype, and 1 had the HPS-4 subtype.

CONCLUSIONS

There is an increased frequency of colitis in our population of 122 HPS patients (8/122, 7%) and in HPS patients referred specifically for symptom evaluation (8/24, 33%). Colitis was found in patients with HPS-1 and HPS-4 genotypes.

High Frequency of Hermansky–Pudlak Syndrome Type 1 (HPS1) Among Japanese Albinism Patients and Functional Analysis of HPS1 Mutant Protein

Hermansky-Pudlak syndrome (HPS) is an autosomal recessive disorder characterized by oculocutaneous albinism (OCA), bleeding tendency, and lysosomal accumulation of ceroid-like material. Seven genetically distinct subtypes of HPS are known in humans; most are rare outside of Puerto Rico. Here, we describe the analysis of the HPS1 gene in 24 Japanese OCA patients who lacked mutations in the four genes known to cause OCA (TYR/OCA1, P/OCA2, TYRP1/OCA3, and MATP/OCA4), and the identification of eight different HPS1 mutations in ten of these patients, four of which were novel (W583X, L668P, 532insC, 1691delA). An IVS5+5G --> A splice consensus mutation was particularly frequent, the result of a founder effect for this allele in Japanese patients. Functional analysis by transfection of the L668P variant into Hps1-mutant melan-ep mouse melanocytes showed that this missense substitution is pathologic, resulting in an Hps-1 protein that is unable to assemble into the biogenesis of lysosome-related organelles complex-3.

Association of the Hermansky-Pudlak syndrome type-3 protein with clathrin

Background

Hermansky-Pudlak syndrome (HPS) is a disorder of lysosome-related organelle biogenesis characterized by oculocutaneous albinism and prolonged bleeding. These clinical findings reflect defects in the formation of melanosomes in melanocytes and dense bodies in platelets. HPS type-3 (HPS-3) results from mutations in the HPS3 gene, which encodes a 1004 amino acid protein of unknown function that contains a predicted clathrin-binding motif (LLDFE) at residues 172–176.

Results

Clathrin was co-immunoprecipitated by HPS3 antibodies from normal but not HPS3 null melanocytes. Normal melanocytes expressing a GFP-HPS3 fusion protein demonstrated partial co-localization of GFP-HPS3 with clathrin following a 20°C temperature block. GFP-HPS3 in which the predicted clathrin-binding domain of HPS3 was mutated (GFP-HPS3-delCBD) did not co-localize with clathrin under the same conditions. Immunoelectron microscopy of normal melanocytes expressing GFP-HPS3 showed co-localization of GFP-HPS3 with clathrin, predominantly on small vesicles in the perinuclear region. In contrast, GFP-HPS3-delCBD did not co-localize with clathrin and exhibited a largely cytoplasmic distribution.

Conclusion

HPS3 associates with clathrin, predominantly on small clathrin-containing vesicles in the perinuclear region. This association most likely occurs directly via a functional clathrin-binding domain in HPS3. These results suggest a role for HPS3 and its protein complex, BLOC-2, in vesicle formation and trafficking.

Eye movement abnormalities in hermansky-pudlak syndrome

BACKGROUND

Hermansky-Pudlak Syndrome (HPS) is a type of oculocutaneous albinism associated with a bleeding diathesis and pulmonary fibrosis. Although it is known that patients with HPS exhibit nystagmus, the nature of these abnormal eye movements has not been studied.

METHODS

Twenty-seven patients with HPS, diagnosed by platelet morphology and genetic analysis, underwent a systemic evaluation and complete eye examination. Twenty-five had eye movement recordings using magnetic search coil, infrared, or video oculography.

RESULTS

All patients had iris transillumination, foveal hypoplasia, and variable hypopigmentation in skin and eyes. All had bleeding tendencies, and 2 reported excessive bleeding during strabismus surgery. Nine patients had pulmonary fibrosis. Visual acuities ranged from 20/20- to 20/320. Twenty patients had strabismus despite 6 having strabismus surgery previously. Ocular oscillations consistent with congenital nystagmus (CN) were clinically evident in 24 of 27 patients, and half showed periodic alternating nystagmus. In 3 patients without CN, eye movement recordings revealed minimal end-gaze nystagmus, square-wave jerks, drift during fixation and saccades, and low-gain pursuit. These patients had melanin in the posterior pole and better visual acuities than the others (P = 0.002).

CONCLUSIONS

Most patients with HPS have CN, and many have periodic alternating nystagmus. Some have subtle eye movement abnormalities without clinically evident nystagmus, which can obscure the diagnosis, especially if hypopigmentation is mild. Absence of clinical nystagmus in a child with HPS suggests good vision. Patients with albinism, especially before surgery, should be evaluated for HPS to prevent life-threatening complications.

Lung pathology of pale ear mouse (model of Hermansky-Pudlak syndrome 1) and beige mouse (model of Chediak-Higashi syndrome): severity of giant lamellar body degeneration of type II pneumocytes correlates with interstitial inflammation

The authors have recently reported the presence of characteristic foamy swelling/degeneration (giant lamellar body degeneration, GLBD) of type II pneumocytes in the lungs affected by Hermansky-Pudlak syndrome (HPS)-associated interstitial pneumonia (HPSIP), and proposed the hypothesis that GLBD may be the triggering factor in the development of HPSIP (Virchows Arch 2000; 437: 304-13). The purpose of the present paper was to investigate the lung pathology of pale ear (ep) mouse, a mouse model of HPS1, and of beige (bg) mouse, a mouse model of Chediak-Higashi syndrome (CHS) with a reference to GLBD and associated pathologic changes. GLBD was found in both ep and bg mice soon after birth, and increased in severity as the mice grew older. Younger mice had only GLBD with no evidence of interstitial change. Aged bg mice showed the most prominent GLBD and patchy areas of alveolar collapse accompanied by lymphocytic infiltration and slight fibrosis. Aged ep mice with less severe GLBD than bg mice of comparable ages also had a slight tendency to develop interstitial inflammation but no fibrosis. The pneumocytes with GLBD were immunoreactive for surfactant protein B and composed of giant lamellar bodies ultrastructurally, findings which were almost identical to those of human GLBD. The results of the present study support the hypothesis that GLBD may play an important role in the development of HPSIP. Ep and bg mice, especially the latter, may be useful mouse models of HPSIP.

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.

Cellular, molecular and clinical characterization of patients with Hermansky-Pudlak syndrome type 5

Hermansky-Pudlak syndrome (HPS) is a disorder of lysosome-related organelles such as melanosomes and platelet dense granules. Seven genes are now associated with HPS in humans. An accurate diagnosis of each HPS subtype has important prognostic and treatment implications. Here we describe the cellular, molecular, and clinical aspects of the recently identified HPS-5 subtype. We first analyzed the genomic organization and the RNA expression pattern of HPS5, located on chromosome 11p14, and demonstrated tissue-specific expression of at least three alternatively spliced HPS5 mRNA transcripts, coding for HPS5A and HPS5B proteins, that differ at their 5'-ends. Genetic screening of 15 unassigned HPS patients yielded six new HPS5 mutations in four patients. Clinically, our HPS-5 patients exhibited iris transillumination, variable hair and skin pigmentation, and absent platelet dense bodies, but not pulmonary fibrosis or granulomatous colitis. In two patients with homozygous missense mutations, hemizygosity was ruled out by gene-dosage multiplex polymerase chain reaction, and immunocytochemical analyses of their fibroblasts supported the HPS-5 diagnosis. Specifically, LAMP-3 distribution was restricted to the perinuclear region in HPS-5 fibroblasts, in contrast to the normal LAMP-3 distribution, which extended to the periphery. This specific intracellular vesicle distribution in fibroblasts, in combination with the clinical features, will improve the characterization of the HPS-5 subtype.

Diverse and essential roles of mammalian vacuolar-type proton pump ATPase: toward the physiological understanding of inside acidic compartments

The vacuolar-type H(+)-ATPases (V-ATPase) are a family of multi-subunit ATP-dependent proton pumps involved in a wide variety of physiological processes. They are present in endomembrane organelles such as vacuoles, lysosomes, endosomes, the Golgi apparatus, chromaffin granules and coated vesicles, and acidify the luminal pH of these intracellular compartments. They also pump protons across the plasma membranes of specialized cells including osteoclasts and epithelial cells in kidneys and male genital tracts. Here, we briefly summarize our recent studies on the diverse and essential roles of mammalian V-ATPase.

Molecular basis of the extreme dilution mottled mouse mutation: a combination of coding and noncoding genomic alterations

Tyrosinase is the rate-limiting enzyme in melanin biosynthesis. It is an N-glycosylated, copper-containing transmembrane protein, whose post-translational processing involves intracytoplasmic movement from the endoplasmic reticulum to the Golgi and, eventually, to the melanosome. The expression of the tyrosinase (Tyr) gene is controlled by several regulatory regions including a locus control region (LCR) located 15 kb upstream from the promoter region. The extreme dilution mottled mutant mice (Tyrc-em) arose spontaneously at the MRC Institute in Harwell (United Kingdom) from a chinchilla-mottled mutant (Tyrc-m) stock, whose molecular basis corresponds to a rearrangement of 5'-upstream regulatory sequences including the LCR of the Tyr gene. Tyrc-em mice display a variegated pigmentation pattern in coat and eyes, in agreement with the LCR translocation, but also show a generalized hypopigmented phenotype, not seen in Tyrc-m mice. Genomic analyses of Tyrc-em mice showed a C1220T nucleotide substitution within the Tyr encoding region, resulting in a T373I amino acid change, which abolishes an N-glycosylation sequon located in the second metal ion binding site of the enzyme. Tyrosinase from Tyrc-em displayed a reduced enzymatic activity in vivo and in vitro, compared with wild-type enzyme. Deglycosylation studies showed that the mutant protein has an abnormal glycosylation pattern and is partially retained in the endoplasmic reticulum. We conclude that the phenotype of the extreme dilution mottled mouse mutant is caused by a combination of coding and noncoding genomic alterations resulting in several abnormalities that include suboptimal gene expression, abnormal protein processing, and reduced enzymatic activity.

Reduced pigmentation (rp), a mouse model of Hermansky-Pudlak syndrome, encodes a novel component of the BLOC-1 complex

Hermansky-Pudlak syndrome (HPS), a disorder of organelle biogenesis, affects lysosomes, melanosomes, and platelet dense bodies. Seven genes cause HPS in humans (HPS1-HPS7) and at least 15 nonallelic mutations cause HPS in mice. Where their function is known, the HPS proteins participate in protein trafficking and vesicle docking/fusion events during organelle biogenesis. HPS-associated genes participate in at least 4 distinct protein complexes: the adaptor complex AP-3; biogenesis of lysosome-related organelles complex 1 (BLOC-1), consisting of 4 HPS proteins (pallidin, muted, cappuccino, HPS7/sandy); BLOC-2, consisting of HPS6/ruby-eye, HPS5/ruby-eye-2, and HPS3/cocoa; and BLOC-3, consisting of HPS1/pale ear and HPS4/light ear. Here, we report the cloning of the mouse HPS mutation reduced pigmentation (rp). We show that the wild-type rp gene encodes a novel, widely expressed 195-amino acid protein that shares 87% amino acid identity with its human orthologue and localizes to punctate cytoplasmic structures. Further, we show that phosphorylated RP is part of the BLOC-1 complex. In mutant rp/rp mice, a premature stop codon truncates the protein after 79 amino acids. Defects in all the 5 known components of BLOC-1, including RP, cause severe HPS in mice, suggesting that the subunits are nonredundant and that BLOC-1 plays a key role in organelle biogenesis.

Lightoid and Claret: a rab GTPase and its putative guanine nucleotide exchange factor in biogenesis of Drosophila eye pigment granules

To elucidate the biogenetic pathways for the generation of lysosome-related organelles, we have chosen to study the Drosophila eye pigment granules because they are lysosome-related and the fruit fly provides the advantages of a genetic system in which many mutations affect eye color. Here, we report the molecular identification of two classic Drosophila eye-color genes required for pigment granule biogenesis, claret and lightoid; the former encodes a protein containing seven repeats with sequence similarity to those that characterize regulator of chromosome condensation 1 (RCC1, a guanine nucleotide exchange factor for the small GTPase, Ran), and the latter encodes a rab GTPase, Rab-RP1. We demonstrate in transfected cells that Claret, through its RCC1-like domain, interacts preferentially with the nucleotide-free form of Rab-RP1, and this interaction involves Claret's first three RCC1-like repeats that are also critical for Claret's function in pigment granule biogenesis in transgenic rescue experiments. In addition, double-mutant analyses suggest that the gene products of claret and lightoid function in the same pathway, which is different from that of garnet and ruby (which encode the delta- and beta-subunit of the tetrameric adaptor protein 3 complex, respectively). Taken together, our results suggest that Claret functions as a guanine nucleotide exchange factor for Lightoid/Rab-RP1 in an adaptor protein 3-independent vesicular trafficking pathway of pigment granule biogenesis.

Milder ocular findings in Hermansky–Pudlak syndrome type 3 compared with Hermansky–Pudlak syndrome type 1

PURPOSE

To compare clinically 2 different subtypes of Hermansky-Pudlak syndrome (HPS), type 1 (HPS-1) and type 3 (HPS-3).

DESIGN

Cross-sectional study of a series of patients.

PARTICIPANTS

Sixteen patients with HPS-1 and 14 patients with HPS-3 were studied.

METHODS

Complete eye examination, including best-corrected visual acuity and photographs and photographic grading of iris transillumination and macular transparency using a previously established grading system.

RESULTS

Snellen visual acuity was 20/160-2 in the HPS-1 group and 20/125+2 in the HPS-3 group (P = 0.017). Iris grading was statistically significant for less translucence in the HPS-3 patients. The HPS-3 patients also tended to have less transparent maculas, but the difference was not statistically significant.

CONCLUSIONS

Patients with HPS-3 have less severe ophthalmic manifestations than patients with HPS-1. Ophthalmologists treating patients with albinism should consider HPS in their differential diagnosis even in the case of mild iris and macular hypopigmentation.

Myospryn Is a Novel Binding Partner for Dysbindin in Muscle

Dysbindin is a coiled-coil-containing protein that was initially identified in a screen for dystrobrevin-interacting proteins. Recently, dysbindin has been shown to be involved in the biogenesis of lysosome-related organelles and is also a major schizophrenia susceptibility factor. Although dysbindin has been implicated in a number of different cellular processes, little is known about its function. To determine the function of dysbindin in muscle, we performed a yeast two-hybrid screen to identify potential interacting proteins. Here we show that dysbindin binds to a novel 413-kDa protein, myospryn, which is expressed in cardiac and skeletal muscle. The transcript encoding myospryn encompasses genethonin-3, a transcript that is down-regulated in muscle from Duchenne muscular dystrophy patients and stretch-responsive protein 553, which is up-regulated in experimental muscle hypertrophy. The C terminus of myospryn contains BBC, FN3, and SPRY domains in a configuration reminiscent of the tripartite motif protein family, as well as the dysbindin-binding site and a region mediating self-association. Dysbindin and myospryn co-immunoprecipitate from muscle extracts and are extensively co-localized. These data demonstrate for the first time that there are tissue-specific ligands for dysbindin that may play important roles in the different disease states involving this protein.

Characterization of BLOC-2, a Complex Containing the Hermansky-Pudlak Syndrome Proteins HPS3, HPS5 and HPS6

Hermansky-Pudlak syndrome (HPS) defines a group of at least seven autosomal recessive disorders characterized by albinism and prolonged bleeding due to defects in the lysosome-related organelles, melanosomes and platelet-dense granules, respectively. Most HPS genes, including HPS3, HPS5 and HPS6, encode ubiquitously expressed novel proteins of unknown function. Here, we report the biochemical characterization of a stable protein complex named Biogenesis of Lysosome-related Organelles Complex-2 (BLOC-2), which contains the HPS3, HPS5 and HPS6 proteins as subunits. The endogenous HPS3, HPS5 and HPS6 proteins from human HeLa cells coimmunoprecipitated with each other from crude extracts as well as from fractions resulting from size-exclusion chromatography and density gradient centrifugation. The native molecular mass of BLOC-2 was estimated to be 340 +/- 64 kDa. As inferred from the biochemical properties of the HPS6 subunit, BLOC-2 exists in a soluble pool and associates to membranes as a peripheral membrane protein. Fibroblasts deficient in the BLOC-2 subunits HPS3 or HPS6 displayed normal basal secretion of the lysosomal enzyme beta-hexosaminidase. Our results suggest a common biological basis underlying the pathogenesis of HPS-3, -5 and -6 disease.

Hermansky-Pudlak syndrome type 4 in a patient from Sri Lanka with pulmonary fibrosis

Hermansky-Pudlak syndrome (HPS) is a rare autosomal recessive disorder characterized by oculocutaneous albinism and a platelet storage pool deficiency. Some patients also develop fatal pulmonary fibrosis and some have granulomatous colitis. Six human genes HPS1, ADB3A, HPS3, HPS4, HPS5, and HPS6 have been identified as cause of the six known subtypes of HPS. While there exist nearly 500 Puerto Rican and non-Puerto Rican HPS-1 patients, very few HPS-4 patients have been reported, and most of these have not been described in detail. We now delineate the clinical characteristics of an HPS-4 patient homozygous for a novel HPS-4 mutation, P685delC. The patient, the first individual with HPS reported from Sri Lanka, had severe pulmonary fibrosis, typical of HPS-1 disease, without granulomatous colitis. We conclude that pulmonary fibrosis occurs as part of HPS-4 and that HPS should be considered in all ethnic groups.

Regulation of Tyrosinase Processing and Trafficking by Organellar pH and by Proteasome Activity

Pigmentation of the hair, skin, and eyes of mammals results from a number of melanocyte-specific proteins that are required for the biosynthesis of melanin. Those proteins comprise the structural and enzymatic components of melanosomes, the membrane-bound organelles in which melanin is synthesized and deposited. Tyrosinase (TYR) is absolutely required for melanogenesis, but other melanosomal proteins, such as TYRP1, DCT, and gp100, also play important roles in regulating mammalian pigmentation. However, pigmentation does not always correlate with the expression of TYR mRNA/protein, and thus its function is also regulated at the post-translational level. Thus, TYR does not necessarily exist in a catalytically active state, and its post-translational activation could be an important control point for regulating melanin synthesis. In this study, we used a multidisciplinary approach to examine the processing and sorting of TYR through the endoplasmic reticulum (ER), Golgi apparatus, coated vesicles, endosomes and early melanosomes because those organelles hold the key to understanding the trafficking of TYR to melanosomes and thus the regulation of melanogenesis. In pigmented cells, TYR is trafficked through those organelles rapidly, but in amelanotic cells, TYR is retained within the ER and is eventually degraded by proteasomes. We now show that TYR can be released from the ER in the presence of protonophore or proton pump inhibitors which increase the pH of intracellular organelles, after which TYR is transported correctly to the Golgi, and then to melanosomes via the endosomal sorting system. The expression of TYRP1, which facilitates TYR processing in the ER, is down-regulated in the amelanotic cells; this is analogous to a hypopigmentary disease known as oculocutaneous albinism type 3 and further impairs melanin production. The sum of these results shows that organellar pH, proteasome activity, and down-regulation of TYRP1 expression all contribute to the lack of pigmentation in TYR-positive amelanotic melanoma cells.

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.

The adaptor protein AP-3 is required for CD1d-mediated antigen presentation of glycosphingolipids and development of Valpha14i NKT cells

Relatively little is known about the pathway leading to the presentation of glycolipids by CD1 molecules. Here we show that the adaptor protein complex 3 (AP-3) is required for the efficient presentation of glycolipid antigens that require internalization and processing. AP-3 interacts with mouse CD1d, and cells from mice deficient for AP-3 have increased cell surface levels of CD1d and decreased expression in late endosomes. Spleen cells from AP-3-deficient mice have a reduced ability to present glycolipids to natural killer T (NKT) cells. Furthermore, AP-3-deficient mice have a significantly reduced NKT cell population, although this is not caused by self-tolerance that might result from increased CD1d surface levels. These data suggest that the generation of the endogenous ligand that selects NKT cells may also be AP-3 dependent. However, the function of MHC class II-reactive CD4+ T lymphocytes is not altered by AP-3 deficiency. Consistent with this divergence from the class II pathway, NKT cell development and antigen presentation by CD1d are not reduced by invariant chain deficiency. These data demonstrate that the AP-3 requirement is a particular attribute of the CD1d pathway in mice and that, although MHC class II molecules and CD1d are both found in late endosomes or lysosomes, different pathways mediate their intracellular trafficking.

Lytic granules, secretory lysosomes and disease

Lytic granules harbour many of the dangerous apoptosis-inducing molecules of the immune system, including perforin, granzymes and Fas ligand. Safe transport, storage and release of these lytic components is vital. As a secretory lysosome, the lytic granule is able to accomplish these roles, as well as conferring the lysosomal functions of cytotoxic T lymphocytes and natural killer cells. Secretory lysosomes are common to many other haemopoietic cells and also melanocytes. Many of the proteins used in lysosomal secretion are found in both melanocytes and hemopoietic cells, and are dysfunctional in genetic diseases with defects in these proteins. The genetically heterogeneous Hermansky-Pudlak syndrome represents an excellent model for revealing proteins involved in secretory lysosome functioning. However, studies of this disease reveal differences between the various different types of secretory lysosomes, including lytic granules.

Lysosome and lysosome-related organelles responsible for specialized functions in higher organisms, with special emphasis on vacuolar-type proton ATPase

Mammals contain various cells differentiated in both morphology and function, which play vital roles in tissue-specific functions. Late endosome/lysosome and lysosomal-related organelles are involved in these specialized functions including antigen presentation, bone remodeling and hormone regulation. To fulfill these diverse roles, lysosomes are present at different levels in different tissues and cell types; however, their morphology within these different tissues varies and the regulation of their activities differs with lysosomal compartments in some cells also functioning as secretory compartments. The luminal acidification of these organelles is closely correlated with their functions. This review will discuss the functions of lysosomes and lysosomal-related organelles, with particular emphasis on the major proton pump, the vacuolar-type proton ATPase (V-ATPase), which is responsible for luminal acidification.

Molecular, ultrastructural and functional characterization of a Spanish family with Hermansky-Pudlak syndrome: role of insC974 in platelet function and clinical relevance

Hermansky-Pudlak syndrome (HPS) is a rare autosomal recessive disorder, which is genetically heterogeneous. In humans, mutations associated with this syndrome have been identified that affect four genes, most of them located in the HPS-1 gene. We evaluated the clinical, molecular, platelet ultrastructure and platelet function data obtained from one Spanish HPS patient and his relatives. The proband was compound heterozygous for a de novo nonsense mutation (Arg-131Stop), which has not been described previously, and for a common frameshift mutation (insC974). These two mutations were also identified by reverse transcription polymerase chain reaction (RT-PCR) in half the RNA, supporting the premise that they have minor effects on either transcription or RNA stability. The patient had an almost complete absence of platelet-dense granules. Accordingly, his platelets showed a small aggregatory response, reduced CD63 surface expression after platelet activation and minor serotonin uptake. Interestingly, despite the absence of clinical symptoms, two relatives carrying only one HPS-1 mutation (insC974) presented a decreased content of platelet-dense granules and showed significant reductions in platelet aggregation, expression of CD63 after platelet activation and serotonin uptake. Data show that the presence of a single mutation affecting one allele of the HPS-1 gene might have relevance in the organogenesis of platelet-dense granules, affecting platelet function. However, these functional defects were not of a great enough magnitude to have clinical significance and, thus, these subjects were clinically asymptomatic.

Hermansky-Pudlak syndrome type 4 (HPS-4): clinical and molecular characteristics

Hermansky-Pudlak syndrome (HPS) is an autosomal recessive disorder of oculocutaneous albinism and bleeding attributable to storage-pool-deficient platelets. Although at least 14 mouse models of HPS exist, the human disorders that comprise HPS, i.e., HPS-1, HPS-2, HPS-3, and HPS-4, are recognized to result from mutations in four genes, viz., HPS1, ADTB3A, HPS3, and HPS4, respectively. To characterize further the recently identified HPS-4 disease on molecular and clinical grounds, we first identified the genomic organization of HPS4, located on chromosome 22q11.2-q12.2, including its intron/exon boundaries. We found that HPS4 produces at least two alternatively spliced mRNA transcripts that differ at their 5'-ends. Next, we performed an extensive analysis of 22 unassigned HPS patients (i.e., not having HPS-1, HPS-2, or HPS-3 disease). Using single-strand conformation polymorphism, we determined that seven of the 22 patients had HPS-4. In these seven individuals, we identified five different HPS4 mutations, including one frameshift insertion, one missense, and three nonsense mutations. Three alleles in two patients contained the previously reported Q698insAAGCA frameshift. Three HPS4 mutations were newly described. Four alleles in three patients contained R217X, and two siblings were compound heterozygotes for E138X and E222X. Clinically, our HPS-4 patients exhibited iris transillumination, variable hair and skin pigmentation, absent platelet dense bodies, and occasional pulmonary fibrosis and granulomatous colitis, a severe phenotype similar to that of patients with HPS-1.

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 Seiji memorial lecture: the melanosome: an ideal model to study cellular differentiation

Melanosomes provide an intriguing model for study at many levels. In part this is due to their unique structure and function, but also in part to their involvement in pigmentary diseases and as a model to study basic cellular mechanisms of organelle biogenesis. Recent studies have elucidated the full proteome of the melanosome and the metabolic and molecular lesions involved in a number of pigmentary diseases have been resolved. This paper summarizes recent advances in the field in these areas.

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.

Biogenesis of lamellar bodies, lysosome-related organelles involved in storage and secretion of pulmonary surfactant

Lamellar bodies are members of a subclass of lysosome-related organelles referred to as secretory lysosomes. The principal constituents of the lamellar body, surfactant phospholipids, are organized into tightly packed, bilayer membranes in a process that is strongly influenced by the lung-specific, hydrophobic peptide SP-B. Newly synthesized SP-B is transported from the Golgi to the lamellar body via multivesicular bodies; in contrast, preliminary evidence suggests that newly synthesized surfactant phospholipids are transported from the ER and incorporated into the internal membranes of the lamellar body via a distinct pathway.

The molecular machinery for the biogenesis of lysosome-related organelles: lessons from Hermansky-Pudlak syndrome

Hermansky-Pudlak syndrome (HPS) defines a group of autosomal recessive disorders characterized by defects in lysosome-related organelles such as melanosomes and platelet dense granules. The genes that are defective in each of the different forms of HPS in humans, or in HPS-like disorders in mice, are thought to encode components of a putative molecular machinery required for the formation of specialized organelles of the lysosomal system. This review discusses the biochemical and functional properties of the products of identified HPS genes, which include subunits of the AP-3 complex and the novel proteins HPS1p, HPS3p, HPS4p, pallidin and muted.

Proteomic analysis of early melanosomes: identification of novel melanosomal proteins

Melanin is a heterogeneous biopolymer produced only by specific cells termed melanocytes, which synthesize and deposit the pigment in specialized membrane-bound organelles known as melanosomes. Although melanosomes have been suspected of being closely related to lysosomes and platelets, the total number of melanosomal proteins is still unknown. Thus far, six melanosome-specific proteins have been identified, and the challenge is to characterize the complete proteome of the melanosome to further understand its mechanism of biogenesis. In this report, we used mass spectrometry and subcellular fractionation to identify protein components of early melanosomes. Using this approach, we have identified all 6 of the known melanosome-specific proteins, 56 proteins that are shared with other organelles, and confirmed the presence of 6 novel melanosomal proteins using western blotting and by immunohistochemistry.

Protein prenyltransferases

SUMMARY

Three different protein prenyltransferases (farnesyltransferase and geranylgeranyltransferases I and II) catalyze the attachment of prenyl lipid anchors 15 or 20 carbons long to the carboxyl termini of a variety of eukaryotic proteins. Farnesyltransferase and geranylgeranyltransferase I both recognize a 'Ca1a2X' motif on their protein substrates; geranylgeranyltransferase II recognizes a different, non-CaaX motif. Each enzyme has two subunits. The genes encoding CaaX protein prenyltransferases are considerably longer than those encoding non-CaaX subunits, as a result of longer introns. Alternative splice forms are predicted to occur, but the extent to which each splice form is translated and the functions of the different resulting isoforms remain to be established. Farnesyltransferase-inhibitor drugs have been developed as anti-cancer agents and may also be able to treat several other diseases. The effects of these inhibitors are complicated, however, by the overlapping substrate specificities of geranylgeranyltransferase I and farnesyltransferase.

Lysosome-related organelles: a view from immunity and pigmentation

Lysosomes are ubiquitous organelles that carry out essential household functions. Certain cell types, however, contain lysosome-related organelles with specialized functions. Their specialized functions are usually reflected by specific morphological and compositional features. A number of diseases that develop due to genetic mutations, pathogen exposure or cell transformation are characterized by dysfunctional lysosomes and/or lysosome-related organelles. In this review we highlight adaptations and malfunction of the endosomal/lysosomal system in normal and pathological situations with special focus on MHC class II compartments in antigen presenting cells and melanosomes in pigment cells.

Hermansky-Pudlak syndrome: vesicle formation from yeast to man

The disorders known as Hermansky-Pudlak syndrome (HPS) are a group of genetic diseases resulting from abnormal formation of intracellular vesicles. In HPS, dysfunction of melanosomes results in oculocutaneous albinism, and absence of platelet dense bodies causes a bleeding diathesis. In addition, some HPS patients suffer granulomatous colitis or fatal pulmonary fibrosis, perhaps due to mistrafficking of a subset of lysosomes. The impaired function of specific organelles indicates that the causative genes encode proteins operative in the formation of certain vesicles. Four such genes, HPS1, ADTB3A, HPS3, and HPS4, are associated with the four known subtypes of HPS, i.e. HPS-1, HPS-2, HPS-3, and HPS-4. ADTB3A codes for the beta 3 A subunit of adaptor complex-3, known to assist in vesicle formation from the trans-Golgi network or late endosome. However, the functions of the HPS1, HPS3, and HPS4 gene products remain unknown. These three genes arose with the evolution of mammals and have no homologs in yeast, reflecting their specialized function. In contrast, all four known HPS-causing genes have homologs in mice, a species with 14 different models of HPS, i.e. hypopigmentation and a platelet storage pool deficiency. Pursuit of the mechanism of mammalian vesicle formation and trafficking, impaired in HPS, relies upon investigation of these mouse models as well as studies of protein complexes involved in yeast vacuole formation.

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.

Hermansky-Pudlak syndrome: radiography and CT of the chest compared with pulmonary function tests and genetic studies

OBJECTIVE

The objective of our study was to describe the chest radiographic and high-resolution CT findings in patients with Hermansky-Pudlak syndrome and to correlate the radiologic findings with age, causative gene, and pulmonary function.

SUBJECTS AND METHODS

Sixty-seven patients with Hermansky-Pudlak syndrome underwent high-resolution CT of the chest. A scoring system based on the extent of pulmonary involvement and specific high-resolution CT findings was used, and the findings were compared with patient age and the results of pulmonary function and genetic studies. Fifty-eight (87%) of the 67 patients also underwent chest radiography. These radiographs were compared with the high-resolution CT scans.

RESULTS

High-resolution CT was more sensitive than chest radiography in evaluating the extent of pulmonary disease in patients with Hermansky-Pudlak syndrome. All patients with mild findings on high-resolution CT scans had normal findings on chest radiographs. Common chest radiographic findings included reticulonodular interstitial pattern, perihilar fibrosis, and pleural thickening. High-resolution CT showed septal thickening, ground-glass opacities, and peribronchovascular thickening. For patients with Hermansky-Pudlak syndrome who were 30 years old or younger, high-resolution CT findings were usually minimal. Among patients who were older than 30 years, the 34 patients with HPS1 mutations had a score of 1.38+/-0.18 (mean+/-standard error of the mean) on high-resolution CT. This score is significantly greater than the score for the 11 patients without HPS1 mutations (0.36 +/- 0.15) (p < 0.001). The score based on high-resolution CT findings inversely correlated with percentage of forced vital capacity and was useful in defining the progression of interstitial disease.

CONCLUSION

High-resolution CT provides a good radiologic monitor of disease status and progression in patients with Hermansky-Pudlak syndrome and correlates well with patient age, extent of pulmonary dysfunction, and genetic findings.

Pallidin is a component of a multi-protein complex involved in the biogenesis of lysosome-related organelles

The Hermansky-Pudlak syndrome defines a group of genetic disorders characterized by defective lysosome-related organelles such as melanosomes and platelet dense bodies. Hermansky-Pudlak syndrome can be caused by mutations of at least four genes in humans and 15 genes in mice. One of these genes is mutated in the pallid mouse strain and encodes a novel protein named pallidin (L. Huang, Y. M. Kuo and J. Gitschier, Nat Genet 1999; 23: 329-332). Pallidin has no homology to any other known protein and no recognizable functional motifs. We have conducted a biochemical characterization of human pallidin using a newly developed polyclonal antibody. We show that pallidin is a ubiquitously expressed approximately 25 kDa protein found both in the cytosol and peripherally associated to membranes. Sedimentation velocity analyses show that native pallidin has a sedimentation coefficient of approximately 5.1 S, much larger than expected from the molecular mass of the pallidin polypeptide. In line with this observation, cosedimentation and coprecipitation analyses reveal that pallidin is part of a hetero-oligomeric complex. One of the subunits of this complex is the product of another Hermansky-Pudlak syndrome gene, muted. Fibroblasts derived from the muted mouse strain exhibit reduced levels of pallidin, suggesting that the absence of the muted protein destabilizes pallidin. These observations indicate that pallidin is a subunit of a novel multi-protein complex involved in the biogenesis of lysosome-related organelles.

BLOC-1, a novel complex containing the pallidin and muted proteins involved in the biogenesis of melanosomes and platelet-dense granules

Recent studies have led to the identification of a group of genes required for normal biogenesis of lysosome-related organelles such as melanosomes and platelet-dense granules. Two of these genes, which are defective in the pallid and muted mutant mouse strains, encode small, coiled-coil-forming proteins that display no homology to each other or to any known protein. We report that these two proteins, pallidin and muted, are components of a novel protein complex. We raised antibodies that allow for detection of pallidin from a wide variety of mammalian cells. Endogenous pallidin was distributed in both soluble and peripheral membrane protein fractions. Size-exclusion chromatography and sedimentation velocity analyses indicated that the bulk of cytosolic pallidin is a component of an asymmetric protein complex with a molecular mass of approximately 200 kDa. We named this complex BLOC-1 (for biogenesis of lysosome-related organelles complex 1). Steady-state pallidin protein levels were reduced in fibroblasts derived from muted and reduced pigmentation mice, suggesting that the genes defective in these two mutant strains could encode components of BLOC-1 that are required for pallidin stability. Co-immunoprecipitation and immunodepletion experiments using an antibody to muted confirmed that this protein is a subunit of BLOC-1. Yeast two-hybrid analyses revealed that pallidin is capable of self-association through a region that contains its two coiled-coil forming domains. Unlike AP-3-deficient pearl fibroblasts, which display defects in intracellular zinc storage, zinc distribution was not noticeably affected in pallid or muted fibroblasts. Interestingly, immunofluorescence and in vitro binding experiments demonstrated that pallidin/BLOC-1 is able to associate with actin filaments. We propose that BLOC-1 mediates the biogenesis of lysosome-related organelles by a mechanism that may involve self-assembly and interaction with the actin cytoskeleton.

Functional redundancy of Rab27 proteins and the pathogenesis of Griscelli syndrome

Griscelli syndrome (GS) patients and the corresponding mouse model ashen exhibit defects mainly in two types of lysosome-related organelles, melanosomes in melanocytes and lytic granules in CTLs. This disease is caused by loss-of-function mutations in RAB27A, which encodes 1 of the 60 known Rab GTPases, critical regulators of vesicular transport. Here we present evidence that Rab27a function can be compensated by a closely related protein, Rab27b. Rab27b is expressed in platelets and other tissues but not in melanocytes or CTLs. Morphological and functional tests in platelets derived from ashen mice are all within normal limits. Both Rab27a and Rab27b are found associated with the limiting membrane of platelet-dense granules and to a lesser degree with alpha-granules. Ubiquitous transgenic expression of Rab27a or Rab27b rescues ashen coat color, and melanocytes derived from transgenic mice exhibit widespread peripheral distribution of melanosomes instead of the perinuclear clumping observed in ashen melanocytes. Finally, transient expression in ashen melanocytes of Rab27a or Rab27b, but not other Rab's, restores peripheral distribution of melanosomes. Our data suggest that Rab27b is functionally redundant with Rab27a and that the pathogenesis of GS is determined by the relative expression of Rab27a and Rab27b in specialized cell types.

Effect of pirfenidone on the pulmonary fibrosis of Hermansky-Pudlak syndrome

Hermansky-Pudlak syndrome (HPS) consists of oculocutaneous albinism, a platelet storage pool deficiency and, in patients with HPS1 gene mutations, a progressive, fatal pulmonary fibrosis. We investigated the safety and efficacy of an antifibrotic agent, pirfenidone (800 mg, t.i.d.), in treating 21 adult Puerto Rican HPS patients, including 20 homozygous for the same HPS1 mutation. Patients were examined every 4 months for up to 44 months in a randomized, placebo-controlled trial, with rate of change in pulmonary function values as outcome parameters. Using the complete data set of 130 patient admissions, a repeated measures model showed that 11 pirfenidone-treated patients lost FVC at a rate 5% of predicted ( approximately 400 mL) per year slower than 10 placebo-treated patients (p=0.001). A random coefficients model showed no significant difference. However, using data restricted to patients with an initial FVC >50% of predicted, both models showed the pirfenidone group losing FVC (p<0.022), FEV(1) (p<0.0007), TLC (p<0.001), and DL(CO) (p<0.122) at a rate approximately 8%/year slower than the placebo group. Clinical and laboratory side effects were similar in the two groups. Pirfenidone appears to slow the progression of pulmonary fibrosis in HPS patients who have significant residual lung function.

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.

The pallidin (Pldn) gene and the role of SNARE proteins in melanosome biogenesis

This review focuses on the product of the pallidin (Pldn) gene, one of a number of genes that in mice are associated with pigmentation defects and platelet dense granule deficiency. A similar combination of defects is also observed in patients suffering from Hermansky-Pudlak (HPS) and Chediak-Higashi (CHS) syndromes. Pldn encodes a novel, approximately 20-kDa protein that is expressed ubiquitously in mammalian tissues. The pallidin protein was found to bind to syntaxin 13, a member of the syntaxin family of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). As SNARE proteins mediate fusion of intracellular membranes, pallidin may play a role in membrane fusion events required for melanosome biogenesis.

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.

The leaden gene product is required with Rab27a to recruit myosin Va to melanosomes in melanocytes

The function of lysosome-related organelles such as melanosomes in melanocytes, and lytic granules in cytotoxic T lymphocytes is disrupted in Griscelli syndrome and related diseases. Griscelli syndrome results from loss of function mutations in either the RAB27A (type 1 Griscelli syndrome) or MYO5A (type 2 Griscelli syndrome) genes. Melanocytes from Griscelli syndrome patients and respective murine models ashen (Rab27a mutant), dilute (myosin Va mutant), and leaden exhibit perinuclear clustering of melanosomes. Recent work suggests that Rab27a is required to recruit myosin Va to melanosomes, thereby tethering melanosomes to the peripheral actin network and promoting melanosome retention at the tips of melanocytic dendrites. Here, we characterize the function of the leaden gene product. We show that Rab27a, but not myosin Va, can be localized to melanosomes in leaden melanocytes, suggesting that the leaden gene product acts downstream of, or in parallel to, Rab27a in melanocytes to promote recruitment of myosin Va to melanosomes. We also observed reduced levels of myosin Va protein in leaden and ashen melanocytes, suggesting that myosin Va stability is influenced by the leaden and ashen gene products. In leaden cytotoxic T lymphocytes, we observed that lytic granules polarize towards the immunological synapse and kill target cells normally. However, in contrast to melanocytes, we found that neither the leaden gene product (melanophilin) nor myosin Va was detectable in cytotoxic T lymphocytes. These results suggest that Rab27a interacts with different classes of effector proteins in melanocytes and cytotoxic T lymphocytes.

The Hermansky-Pudlak syndrome 1 (HPS1) and HPS2 genes independently contribute to the production and function of platelet dense granules, melanosomes, and lysosomes

Hermansky-Pudlak syndrome (HPS) is an inherited hemorrhagic disease affecting the related subcellular organelles platelet dense granules, lysosomes, and melanosomes. The mouse genes for HPS, pale ear and pearl, orthologous to the human HPS1 and HPS2 (ADTB3A) genes, encode a novel protein of unknown function and the β3A subunit of the AP-3 adaptor complex, respectively. To test for in vivo interactions between these genes in the production and function of intracellular organelles, mice doubly homozygous for the 2 mutant genes were produced by appropriate breeding. Cooperation between the 2 genes in melanosome production was evident in increased hypopigmentation of the coat together with dramatic quantitative and qualitative alterations of melanosomes of the retinal pigment epithelium and choroid of double mutant mice. Lysosomal and platelet dense granule abnormalities, including hyposecretion of lysosomal enzymes from kidneys and depression of serotonin concentrations of platelet dense granules were likewise more severe in double than single mutants. Also, lysosomal enzyme concentrations were significantly increased in lungs of double mutant mice. Interaction between the 2 genes was specific in that effects on organelles were confined to melanosomes, lysosomes, and platelet dense granules. Together, the evidence indicates these 2 HPS genes function largely independently at the whole organism level to affect the production and function of all 3 organelles. Further, the increased lysosomal enzyme levels in lung of double mutant mice suggest a cause of a major clinical problem of HPS, lung fibrosis. Finally, doubly mutant HPS mice are a useful laboratory model for analysis of severe HPS phenotypes.

A novel immunocytochemical assay for the detection of serotonin in platelets

A method for the rapid, inexpensive and easy detection of platelet serotonin (5-hydroxytryptamine, 5-HT) is not currently available. Consequently, many patients suffering from unresolved platelet-related bleeding disorders are not examined for a possible platelet 5-HT deficiency. The direct measurement of 5-HT concentration with high-performance liquid chromatography (HPLC) or serotonin enzyme-linked immunosorbent assay (ELISA) is costly and highly demanding. Indirect methods, which determine the content of ATP or calcium with lumi-aggregometry or electron microscopy, rely upon the assumption that the ATP or calcium concentration is equivalent to that of 5-HT. We have developed a fluorescence-based assay for 5-HT that can be performed within 2 h on fresh or frozen samples using a fluorescence microscope or a flow cytometer. The assay requires only 0.2 ml of platelet-rich plasma and might therefore be of particular interest for paediatric patients. Samples from control and patient donors were analysed for 5-HT with the new immunocytochemical assay in comparison with HPLC and/or 5-HT ELISA. Patients with Hermansky-Pudlak syndrome were readily identified. The new assay was also reliable in cases where the 5-HT content of dense granules was not correlated with the calcium or ATP content, such as in calcium deficiency or in the presence of selective serotonin reuptake inhibitors.