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

AP-3-dependent targeting of flippase ATP8A1 to lamellar bodies suppresses activation of YAP in alveolar epithelial type 2 cells

Lamellar bodies (LBs) are lysosome-related organelles (LROs) of surfactant-producing alveolar type 2 (AT2) cells of the distal lung epithelium. Trafficking pathways to LBs have been understudied but are likely critical to AT2 cell homeostasis given associations between genetic defects of endosome to LRO trafficking and pulmonary fibrosis in Hermansky Pudlak syndrome (HPS). Our prior studies uncovered a role for AP-3, defective in HPS type 2, in trafficking Peroxiredoxin-6 to LBs. We now show that the P4-type ATPase ATP8A1 is sorted by AP-3 from early endosomes to LBs through recognition of a C-terminal dileucine-based signal. Disruption of the AP-3/ATP8A1 interaction causes ATP8A1 accumulation in early sorting and/or recycling endosomes, enhancing phosphatidylserine exposure on the cytosolic leaflet. This in turn promotes activation of Yes-activating protein, a transcriptional coactivator, augmenting cell migration and AT2 cell numbers. Together, these studies illuminate a mechanism whereby loss of AP-3-mediated trafficking contributes to a toxic gain-of-function that results in enhanced and sustained activation of a repair pathway associated with pulmonary fibrosis.

The endoplasmic reticulum protein SEC22B interacts with NBEAL2 and is required for megakaryocyte α-granule biogenesis

Studies of inherited platelet disorders have provided many insights into platelet development and function. Loss of function of neurobeachin-like 2 (NBEAL2) causes gray platelet syndrome (GPS), where the absence of platelet α-granules indicates NBEAL2 is required for their production by precursor megakaryocytes. The endoplasmic reticulum is a dynamic network that interacts with numerous intracellular vesicles and organelles and plays key roles in their development. The megakaryocyte endoplasmic reticulum is extensive, and in this study we investigated its role in the biogenesis of α-granules by focusing on the membrane-resident trafficking protein SEC22B. Coimmunoprecipitation (co-IP) experiments using tagged proteins expressed in human HEK293 and megakaryocytic immortalized megakaryocyte progenitor (imMKCL) cells established binding of NBEAL2 with SEC22B, and demonstrated that NBEAL2 can simultaneously bind SEC22B and P-selectin. NBEAL2-SEC22B binding was also observed for endogenous proteins in human megakaryocytes using co-IP, and immunofluorescence microscopy detected substantial overlap. SEC22B binding was localized to a region of NBEAL2 spanning amino acids 1798 to 1903, where 2 GPS-associated missense variants have been reported: E1833K and R1839C. NBEAL2 containing either variant did not bind SEC22B coexpressed in HEK293 cells. CRISPR/Cas9-mediated knockout of SEC22B in imMKCL cells resulted in decreased NBEAL2, but not vice versa. Loss of either SEC22B or NBEAL2 expression resulted in failure of α-granule production and reduced granule proteins in imMKCL cells. We conclude that SEC22B is required for α-granule biogenesis in megakaryocytes, and that interactions with SEC22B and P-selectin facilitate the essential role of NBEAL2 in granule development and cargo stability.

Modeling of Fibrotic Lung Disease Using 3D Organoids Derived from Human Pluripotent Stem Cells

The pathogenesis of idiopathic pulmonary fibrosis (IPF), an intractable interstitial lung disease, is unclear. Recessive mutations in some genes implicated in Hermansky-Pudlak syndrome (HPS) cause HPS-associated interstitial pneumonia (HPSIP), a clinical entity that is similar to IPF. We previously reported that HPS1^-/- embryonic stem cell-derived 3D lung organoids showed fibrotic changes. Here, we show that the introduction of all HPS mutations associated with HPSIP promotes fibrotic changes in lung organoids, while the deletion of HPS8, which is not associated with HPSIP, does not. Genome-wide expression analysis revealed the upregulation of interleukin-11 (IL-11) in epithelial cells from HPS mutant fibrotic organoids. IL-11 was detected predominantly in type 2 alveolar epithelial cells in end-stage IPF, but was expressed more broadly in HPSIP. Finally, IL-11 induced fibrosis in WT organoids, while its deletion prevented fibrosis in HPS4^-/- organoids, suggesting IL-11 as a therapeutic target. hPSC-derived 3D lung organoids are, therefore, a valuable resource to model fibrotic lung disease.

Adaptor protein-3: A key player in RBL-2H3 mast cell mediator release

Mast cell (MC) secretory granules are Lysosome-Related Organelles (LROs) whose biogenesis is associated with the post-Golgi secretory and endocytic pathways in which the sorting of proteins destined for a specific organelle relies on the recognition of sorting signals by adaptor proteins that direct their incorporation into transport vesicles. The adaptor protein 3 (AP-3) complex mediates protein trafficking between the trans-Golgi network (TGN) and late endosomes, lysosomes, and LROs. AP-3 has a recognized role in LROs biogenesis and regulated secretion in several cell types, including many immune cells such as neutrophils, natural killer cells, and cytotoxic T lymphocytes. However, the relevance of AP-3 for these processes in MCs has not been previously investigated. AP-3 was found to be expressed and distributed in a punctate fashion in rat peritoneal mast cells ex vivo. The rat MC line RBL-2H3 was used as a model system to investigate the role of AP-3 in mast cell secretory granule biogenesis and mediator release. By immunofluorescence and immunoelectron microscopy, AP-3 was localized both to the TGN and early endosomes indicating that AP-3 dependent sorting of proteins to MC secretory granules originates in these organelles. ShRNA mediated depletion of the AP-3 δ subunit was shown to destabilize the AP-3 complex in RBL-2H3 MCs. AP-3 knockdown significantly affected MC regulated secretion of β-hexosaminidase without affecting total cellular enzyme levels. Morphometric evaluation of MC secretory granules by electron microscopy revealed that the area of MC secretory granules in AP-3 knockdown MCs was significantly increased, indicating that AP-3 is involved in MC secretory granule biogenesis. Furthermore, AP-3 knockdown had a selective impact on the secretion of newly formed and newly synthesized mediators. These results show for the first time that AP-3 plays a critical role in secretory granule biogenesis and mediator release in MCs.

Bleeding Diatheses: Approach to the Patient Who Bleeds or Has Abnormal Coagulation

Many complex elements contribute to normal hemostasis, and an imbalance of these elements may lead to abnormal bleeding. In addition to evaluating medication effects, the hematologist must evaluate for congenital or acquired deficiencies in coagulation factors and platelet disorders. This evaluation should include a thorough bleeding history with careful attention to prior hemostatic challenges and common laboratory testing, including coagulation studies and/or functional platelet assays. An accurate diagnosis of a bleeding diathesis and selection of appropriate treatment are greatly aided by a basic understanding of the mechanisms of disease and the tests used to diagnose them.

Identification of Atg2 and ArfGAP1 as Candidate Genetic Modifiers of the Eye Pigmentation Phenotype of Adaptor Protein-3 (AP-3) Mutants in Drosophila melanogaster

The Adaptor Protein (AP)-3 complex is an evolutionary conserved, molecular sorting device that mediates the intracellular trafficking of proteins to lysosomes and related organelles. Genetic defects in AP-3 subunits lead to impaired biogenesis of lysosome-related organelles (LROs) such as mammalian melanosomes and insect eye pigment granules. In this work, we have performed a forward screening for genetic modifiers of AP-3 function in the fruit fly, Drosophila melanogaster. Specifically, we have tested collections of large multi-gene deletions–which together covered most of the autosomal chromosomes–to identify chromosomal regions that, when deleted in single copy, enhanced or ameliorated the eye pigmentation phenotype of two independent AP-3 subunit mutants. Fine-mapping led us to define two non-overlapping, relatively small critical regions within fly chromosome 3. The first critical region included the Atg2 gene, which encodes a conserved protein involved in autophagy. Loss of one functional copy of Atg2 ameliorated the pigmentation defects of mutants in AP-3 subunits as well as in two other genes previously implicated in LRO biogenesis, namely Blos1 and lightoid, and even increased the eye pigment content of wild-type flies. The second critical region included the ArfGAP1 gene, which encodes a conserved GTPase-activating protein with specificity towards GTPases of the Arf family. Loss of a single functional copy of the ArfGAP1 gene ameliorated the pigmentation phenotype of AP-3 mutants but did not to modify the eye pigmentation of wild-type flies or mutants in Blos1 or lightoid. Strikingly, loss of the second functional copy of the gene did not modify the phenotype of AP-3 mutants any further but elicited early lethality in males and abnormal eye morphology when combined with mutations in Blos1 and lightoid, respectively. These results provide genetic evidence for new functional links connecting the machinery for biogenesis of LROs with molecules implicated in autophagy and small GTPase regulation.

Defective release of α granule and lysosome contents from platelets in mouse Hermansky-Pudlak syndrome models

Hermansky-Pudlak syndrome (HPS) is characterized by oculocutaneous albinism, bleeding diathesis, and other variable symptoms. The bleeding diathesis has been attributed to δ storage pool deficiency, reflecting the malformation of platelet dense granules. Here, we analyzed agonist-stimulated secretion from other storage granules in platelets from mouse HPS models that lack adaptor protein (AP)-3 or biogenesis of lysosome-related organelles complex (BLOC)-3 or BLOC-1. We show that α granule secretion elicited by low agonist doses is impaired in all 3 HPS models. High agonist doses or supplemental adenosine 5'-diphosphate (ADP) restored normal α granule secretion, suggesting that the impairment is secondary to absent dense granule content release. Intravital microscopy following laser-induced vascular injury showed that defective hemostatic thrombus formation in HPS mice largely reflected reduced total platelet accumulation and affirmed a reduced area of α granule secretion. Agonist-induced lysosome secretion ex vivo was also impaired in all 3 HPS models but was incompletely rescued by high agonist doses or excess ADP. Our results imply that (1) AP-3, BLOC-1, and BLOC-3 facilitate protein sorting to lysosomes to support ultimate secretion; (2) impaired secretion of α granules in HPS, and to some degree of lysosomes, is secondary to impaired dense granule secretion; and (3) diminished α granule and lysosome secretion might contribute to pathology in HPS.

The Ap3b1 gene regulates the ocular melanosome biogenesis and tyrosinase distribution differently from the Hps1 gene

Hermansky-Pudlak syndrome (HPS) is an autosomal recessive disorder in humans and mice. The pearl (pe) mouse, a mouse model for the human HPS-2, bears a mutation in Ap3b1 gene. Here we investigated the pigmentation in eyes of pearl (pe) mice, and compared it with our previously published data in pale ear (ep) mice. We revealed that the hypopigmentation in eyes of pearl mice was more severe than pale ear mice, especially in the neural crest-derived tissues. However, the total tyrosinase activity in eyes of pearl mice was stronger than pale ear mice, suggesting that the degradation of aberrantly transported tyrosinase in eyes of pearl mice was weaker than that of pale ear mice. Furthermore, the pigmentation in eyes of mice doubly heterozygous for Hps1 and Ap3b1 genes was similar to the wild-type, while the hypopigmentation in iris of double mutant mice was more severe than either single mutant. Besides, we found several previously reported characters in pale ear mice, including macromelanosomes in the neural crest-derived melanocytes and increased accumulation of lipofuscin in the RPE, were absent in pearl mice. Our study indicates that Ap3b1 gene play distinct roles in melanin production and tyrosinase distribution compared with Hps1 gene.

Two distinct phenotypes in pigmented cells of different embryonic origins in eyes of pale ear mice

The eye has pigmented cells of two different embryonic origins and therefore it is a good model for studying melanosome biogenesis and melanin production/deposition. Pale ear mice bear a mutation in the Hermansky-Pudlak syndrome type 1 (HPS-1) gene and exhibit abnormal eye pigmentation. Here, we reported the delayed and reduced pigmentation in eyes of pale ear mice in early postnatal stages and adulthood. Tyrosinase assay and L-3,4-dihydroxyphenylalanine (L-DOPA) gel staining assay revealed that tyrosinase activity in eyes of pale ear mutants was greatly reduced in early postnatal stages and increased gradually after postnatal day 7 (P7). Further histological examination revealed that hypopigmentation in the retinal pigment epithelium (RPE) and pigment epithelium of the iris and ciliary body, which are derived from the optic cup, was more severe than that in neural crest-derived tissues. In addition, macromelanosomes were exclusively present in neural crest-derived melanocytes of pale ear adults, but absent at early postnatal stages. Taken together, the mutation in the HPS-1 gene could cause two distinct phenotypes in pigmented cells of different embryonic origins. Besides, an increased accumulation of lipofuscin in RPE was also observed.

Epithelial stress and apoptosis underlie Hermansky-Pudlak syndrome-associated interstitial pneumonia

RATIONALE

The molecular mechanisms underlying Hermansky-Pudlak syndrome-associated interstitial pneumonia (HPSIP) are poorly understood but, as in idiopathic pulmonary fibrosis, may be linked to chronic alveolar epithelial type II cell (AECII) injury.

OBJECTIVES

We studied the development of fibrosis and the role of AECII injury in various murine models of HPS.

METHODS

HPS1, HPS2, and HPS6 monomutant mice, and HPS1/2 and HPS1/6 double-mutant and genetic background mice, were killed at 3 and 9 months of age. Quantitative morphometry was undertaken in lung sections stained with hemalaun-eosin. The extent of lung fibrosis was assessed by trichrome staining and hydroxyproline measurement. Surfactant lipids were analyzed by electrospray ionization mass spectrometry. Surfactant proteins, apoptosis, and lysosomal and endoplasmic reticulum stress markers were studied by Western blotting and immunohistochemistry. Cell proliferation was measured by water-soluble tetrazolium salt-1 and bromodeoxyuridine assays.

MEASUREMENTS AND MAIN RESULTS

Spontaneous and slowly progressive HPSIP was observed in HPS1/2 double mutants, but not in other HPS mutants, with subpleural onset at 3 months and full-blown fibrosis at 9 months. In these mice, extensive surfactant abnormalities were encountered in AECII and were paralleled by early lysosomal stress (cathepsin D induction), late endoplasmic reticulum stress (activating transcription factor-4 [ATF4], C/EBP homologous protein [CHOP] induction), and marked apoptosis. These findings were fully corroborated in human HPSIP. In addition, cathepsin D overexpression resulted in apoptosis of MLE-12 cells and increased proliferation of NIH 3T3 fibroblasts incubated with conditioned medium of the transfected cells.

CONCLUSIONS

Extensively impaired surfactant trafficking and secretion underlie lysosomal and endoplasmic reticulum stress with apoptosis of AECII in HPSIP, thereby causing the development of HPSIP.

Alveolar macrophage dysregulation in Hermansky-Pudlak syndrome type 1

RATIONALE

Individuals with Hermansky-Pudlak syndrome type 1 (HPS-1), an autosomal recessive disorder characterized by defective biogenesis of lysosome-related organelles, develop an accelerated form of progressive fibrotic lung disease. The etiology of pulmonary fibrosis associated with HPS-1 is unknown.

OBJECTIVES

To investigate the potential pathogenesis of pulmonary fibrosis in HPS-1, lung cells and proteins from individuals with HPS-1 were studied.

METHODS

Forty-one subjects with HPS-1 with and without pulmonary fibrosis were evaluated with pulmonary function tests, high-resolution computed tomography scan, and bronchoscopy. Bronchoalveolar lavage cells and analytes were analyzed.

MEASUREMENTS AND MAIN RESULTS

Concentrations of total bronchoalveolar lavage cells and alveolar macrophages were significantly higher in epithelial lining fluid from subjects with HPS-1 with and without pulmonary fibrosis compared with healthy research volunteers. Concentrations of cytokines and chemokines (i.e., monocyte chemoattractant protein-1, macrophage inflammatory protein-1alpha, and granulocyte-macrophage colony-stimulating factor) in alveolar epithelial lining fluid were significantly higher in subjects with HPS-1 with and without pulmonary fibrosis compared with healthy research volunteers (P < 0.001). In vitro, HPS-1 pulmonary fibrosis alveolar macrophages, which did not express HPS1 mRNA, secreted significantly higher concentrations of monocyte chemoattractant protein-1, macrophage inflammatory protein-1alpha, and regulated upon activation, normal T cell expressed and secreted (RANTES) protein compared with normal cells (P = 0.001, P = 0.014, and P = 0.011, respectively). Pirfenidone suppressed HPS-1 alveolar macrophage cytokine and chemokine secretion in vitro in a dose-dependent manner.

CONCLUSIONS

In HPS-1, alveolar inflammation predominantly involves macrophages and is associated with high lung concentrations of cytokines and chemokines. HPS-1 alveolar macrophages provide a model system in which to study the pathogenesis and treatment of HPS pulmonary fibrosis.

The BLOC interactomes form a network in endosomal transport

With the identification of more than a dozen novel Hermansky-Pudlak Syndrome (HPS) proteins in vesicle trafficking in higher eukaryotes, a new class of trafficking pathways has been described. It mainly consists of three newly-defined protein complexes, BLOC-1, -2, and -3. Compelling evidence indicates that these complexes together with two other well-known complexes, AP3 and HOPS, play important roles in endosomal transport. The interactions between these complexes form a network in protein trafficking via endosomes and cytoskeleton. Each node of this network has intra-complex and extra-complex interactions. These complexes are connected by direct interactions between the subunits from different complexes or by indirect interactions through coupling nodes that interact with two or more subunits from different complexes. The dissection of this network facilitates the understanding of a dynamic but elaborate transport machinery in protein/membrane trafficking. The disruption of this network may lead to abnormal trafficking or defective organellar development as described in patients with Hermansky-Pudlak syndrome.

Dysbindin-1, a schizophrenia-related protein, functionally interacts with the DNA- dependent protein kinase complex in an isoform-dependent manner

DTNBP1 has been recognized as a schizophrenia susceptible gene, and its protein product, dysbindin-1, is down-regulated in the brains of schizophrenic patients. However, little is known about the physiological role of dysbindin-1 in the central nervous system. We hypothesized that disruption of dysbindin-1 with unidentified proteins could contribute to pathogenesis and the symptoms of schizophrenia. GST pull-down from human neuroblastoma lysates showed an association of dysbindin-1 with the DNA-dependent protein kinase (DNA-PK) complex. The DNA-PK complex interacts only with splice isoforms A and B, but not with C. We found that isoforms A and B localized in nucleus, where the kinase complex exist, whereas the isoform C was found exclusively in cytosol. Furthermore, results of phosphorylation assay suggest that the DNA-PK complex phosphorylated dysbindin-1 isoforms A and B in cells. These observations suggest that DNA-PK regulates the dysbindin-1 isoforms A and B by phosphorylation in nucleus. Isoform C does not contain exons from 1 to 6. Since schizophrenia-related single nucleotide polymorphisms (SNPs) occur in these introns between exon 1 and exon 6, we suggest that these SNPs might affect splicing of DTNBP1, which leads to impairment of the functional interaction between dysbindin-1 and DNA-PK in schizophrenic patients.

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.

Genetic context determines susceptibility to intraocular pressure elevation in a mouse pigmentary glaucoma

BACKGROUND

DBA/2J (D2) mice develop an age-related form of glaucoma. Their eyes progressively develop iris pigment dispersion and iris atrophy followed by increased intraocular pressure (IOP) and glaucomatous optic nerve damage. Mutant alleles of the Gpnmb and Tyrp1 genes are necessary for the iris disease, but it is unknown whether alleles of other D2 gene(s) are necessary for the distinct later stages of disease. We initiated a study of congenic strains to further define the genetic requirements and disease mechanisms of the D2 glaucoma.

RESULTS

To further understand D2 glaucoma, we created congenic strains of mice on the C57BL/6J (B6) genetic background. B6 double-congenic mice carrying D2-derived Gpnmb and Tyrp1 mutations develop a D2-like iris disease. B6 single-congenics with only the Gpnmb and Tyrp1 mutations develop milder forms of iris disease. Genetic epistasis experiments introducing a B6 tyrosinase mutation into the congenic strains demonstrated that both the single and double-congenic iris diseases are rescued by interruption of melanin synthesis. Importantly, our experiments analyzing mice at ages up to 27 months indicate that the B6 double-congenic mice are much less prone to IOP elevation and glaucoma than are D2 mice.

CONCLUSION

As demonstrated here, the Gpnmb and Tyrp1 iris phenotypes are both individually dependent on tyrosinase function. These results support involvement of abnormal melanosomal events in the diseases caused by each gene. In the context of the inbred D2 mouse strain, the glaucoma phenotype is clearly influenced by more genes than just Gpnmb and Tyrp1. Despite the outward similarity of pigment-dispersing iris disease between D2 and the B6 double-congenic mice, the congenic mice are much less susceptible to developing high IOP and glaucoma. These new congenic strains provide a valuable new resource for further studying the genetic and mechanistic complexity of this form of glaucoma.

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.

Insights into the biogenesis of lysosome-related organelles from the study of the Hermansky-Pudlak syndrome

Lysosome-related organelles (LROs) are a family of cell-type-specific organelles that include melanosomes, platelet dense bodies, and cytotoxic T cell granules. The name, LRO, recognizes the fact that all of these organelles contain subsets of lysosomal proteins in addition to cell-type-specific proteins. The recent identification of genetic disorders that cause combined defects in several of these organelles indicates that they share common biogenetic pathways. Studies of one of these disorders, the Hermansky-Pudlak syndrome (HPS), have provided helpful insights into the molecular machinery involved in LRO biogenesis. HPS is a genetically heterogeneous disorder caused by mutations in any of 7 genes in humans and 15 genes in mice. These genes encode subunits of 4 multi-protein complexes named AP-3, BLOC-1, BLOC-2 and BLOC-3, in addition to miscellaneous components of the general protein trafficking machinery. The AP-3 complex is a coat protein involved in vesicle formation and cargo selection in the endosomal-lysosomal system. One of these cargo molecules is the melanosomal enzyme, tyrosinase, the missorting of which may explain the defective melanosomes in AP-3-deficient humans and mice. The function of the BLOC complexes is unknown, although they are thought to mediate either vesicle tethering/fusion or cytoplasmic dispersal of LROs. Further studies of these complexes should contribute to the elucidation of the mechanisms of LRO biogenesis and the pathogenesis of HPS.

The gene expression signatures of melanoma progression

Because of the paucity of available tissue, little information has previously been available regarding the gene expression profiles of primary melanomas. To understand the molecular basis of melanoma progression, we compared the gene expression profiles of a series of nevi, primary melanomas, and melanoma metastases. We found that metastatic melanomas exhibit two dichotomous patterns of gene expression, which unexpectedly reflect gene expression differences already apparent in comparing laser-capture microdissected radial and vertical phases of a large primary melanoma. Unsupervised hierarchical clustering accurately separated nevi and primary melanomas. Multiclass significance analysis of microarrays comparing normal skin, nevi, primary melanomas, and the two types of metastatic melanoma identified 2,602 transcripts that significantly correlated with sample class. These results suggest that melanoma pathogenesis can be understood as a series of distinct molecular events. The gene expression signatures identified here provide the basis for developing new diagnostics and targeting therapies for patients with malignant melanoma.

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.

Intermediate Filaments and Vesicular Membrane Traffic: The Odd Couple's First Dance?

During the last two decades, much attention has been focused on the regulation of membrane traffic by the actin and microtubule cytoskeletal networks. Their dynamic and polarized behavior and associated motors provide a logical framework from which architectural and movement cues can be communicated to organelles. The study of these cytoskeletal systems has been greatly aided by pharmacological agents. In contrast, intermediate filaments (IFs) have largely been neglected as a potential player in membrane traffic, both because a comprehensive pharmacology to perturb them does not exist and because they lack the intrinsic polarity and specific motors that make the other cytoskeletal systems attractive. In this review, we will discuss evidence suggesting that IFs may play roles in controlling organelle positioning and in membrane protein targeting. Furthermore, we will discuss potential mechanisms by which IFs may regulate the localization and function of organelles.

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.

Diagnostic usefulness of bronchoalveolar lavage in Hermansky-Pudlak syndrome: a case with double lung cancers

A 65-year-old man was admitted to our hospital because of dyspnea on exertion. He had oculocutaneous albinism innately and his parents were consanguineous. His chest roentgenogram on admission showed reticulo-nodular infiltrates and cystic changes throughout both lung fields, and 7 cm mass in the left middle field. Cytology of bronchoalveolar lavage fluid (BALF) revealed macrophages containing ceroid. The diagnosis of HPS was made clinically and the tumor was diagnosed as poorly differentiated adenocarcinoma of the lung. He died of respiratory failure. By autopsy, additional well-differentiated adenocarcinoma was detected. Cytology of BALF was useful to confirm ceroid accumulation in the lung.

Identification of genes contributing to the obese yellow Avy phenotype: caloric restriction, genotype, diet x genotype interactions

The incidence and severity of obesity and type 2 diabetes are increasing in Western societies. The progression of obesity to type 2 diabetes is gradual with overlapping symptoms of insulin resistance, hyperinsulinemia, hyperglycemia, dyslipidemias, ion imbalance, and inflammation; this complex syndrome has been called diabesity. We describe here comparisons of gene expression in livers of A/a (agouti) vs. A(vy)/A (obese yellow) segregants (i.e., littermates) from BALB/cStCrlfC3H/Nctr x VYWffC3Hf/Nctr-A(vy)/a matings in response to 70% and 100% of ad libitum caloric intakes of a reproducible diet. Twenty-eight (28) genes regulated by diet, genotype, or diet x genotype interactions mapped to diabesity quantitative trait loci. A subset of the identified genes is linked to abnormal physiological signs observed in obesity and diabetes.

The building BLOC(k)s of lysosomes and related organelles

Hermansky-Pudlak syndrome defines a group of genetic disorders characterized by defects in organelles of the endosomal-lysosomal system, most notably melanosomes and platelet-dense granules. About a dozen genes have been implicated in the pathogenesis of the disease in humans and mice. Most of these genes encode novel polypeptides that are not conserved in unicellular eukaryotes. Recent studies have revealed that these polypeptides are stable components of at least three distinct, ubiquitously expressed protein complexes, named biogenesis of lysosome-related organelles complex (BLOC)-1, -2 and -3. These findings provide a framework for studies on the function of these proteins and the pathogenesis of Hermansky-Pudlak syndrome.

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.

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.

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.

Specific regulation of the adaptor protein complex AP-3 by the Arf GAP AGAP1

Arf1 regulates membrane trafficking at several membrane sites by interacting with at least seven different vesicle coat proteins. Here, we test the hypothesis that Arf1-dependent coats are independently regulated by specific interaction with Arf GAPs. We find that the Arf GAP AGAP1 directly associates with and colocalizes with AP-3, a coat protein complex involved in trafficking in the endosomal-lysosomal system. Binding is mediated by the PH domain of AGAP1 and the delta and sigma3 subunits of AP-3. Overexpression of AGAP1 changes the cellular distribution of AP-3, and reduced expression of AGAP1 renders AP-3 resistant to brefeldin A. AGAP1 overexpression does not affect the distribution of other coat proteins, and AP-3 distribution is not affected by overexpression of other Arf GAPs. Cells overexpressing AGAP1 also exhibit increased LAMP1 trafficking via the plasma membrane. Taken together, these results support the hypothesis that AGAP1 directly and specifically regulates AP-3-dependent trafficking.

Biogenesis of lysosome-related organelles complex 3 (BLOC-3): a complex containing the Hermansky-Pudlak syndrome (HPS) proteins HPS1 and HPS4

Hermansky-Pudlak syndrome (HPS) defines a group of autosomal recessive disorders characterized by deficiencies in lysosome-related organelles such as melanosomes and platelet-dense granules. Several HPS genes encode proteins of unknown function including HPS1, HPS3, and HPS4. Here we have identified and characterized endogenous HPS3 and HPS4 proteins from HeLa cells. Both proteins were found in soluble and membrane-associated forms. Sedimentation-velocity and coimmunoprecipitation experiments revealed that HPS4 but not HPS3 associates with HPS1 in a complex, which we term biogenesis of lysosome-related organelles complex 3 (BLOC-3). Mutant fibroblasts deficient in either HPS1 or HPS4 displayed abnormal localization of lysosomes and late endosomes, which were less concentrated at the juxtanuclear region in mutant cells than in control fibroblasts. The coat-color phenotype of young homozygous double-mutant mice deficient in subunits of BLOC-3 (HPS1) and BLOC-1 (pallidin) was indistinguishable from that of BLOC-1 single mutants. Taken together, these observations suggest that HPS1 and HPS4 are components of a protein complex that regulates the intracellular localization of lysosomes and late endosomes and may function in a BLOC-1-dependent pathway for melanosome biogenesis.

Mutations in the clathrin-assembly gene Picalm are responsible for the hematopoietic and iron metabolism abnormalities in fit1 mice

Recessive N-ethyl-N-nitrosourea (ENU)-induced mutations recovered at the fitness-1 (fit1) locus in mouse chromosome 7 cause hematopoietic abnormalities, growth retardation, and shortened life span, with varying severity of the defects in different alleles. Abnormal iron distribution and metabolism and frequent scoliosis have also been associated with an allele of intermediate severity (fit14R). We report that fit14R, as well as the most severe fit15R allele, are nonsense point mutations in the mouse ortholog of the human phosphatidylinositol-binding clathrin assembly protein (PICALM) gene, whose product is involved in clathrin-mediated endocytosis. A variety of leukemias and lymphomas have been associated with translocations that fuse human PICALM with the putative transcription factor gene AF10. The Picalmfit1-5R and Picalmfit1-4R mutations are splice-donor alterations resulting in transcripts that are less abundant than normal and missing exons 4 and 17, respectively. These exon deletions introduce premature termination codons predicted to truncate the proteins near the N and C termini, respectively. No mutations in the genes encoding Picalm, clathrin, or components of the adaptor protein complex 2 (AP2) have been previously described in which the suite of disorders present in the Picalmfit1 mutant mice is apparent. These mutants thus provide unique models for exploring how the endocytic function of mouse Picalm and the transport processes mediated by clathrin and the AP2 complex contribute to normal hematopoiesis, iron metabolism, and growth.

The fate and function of glycosphingolipid glucosylceramide

In higher eukaryotes, glucosylceramide is the simplest member and precursor of a fascinating class of membrane lipids, the glycosphingolipids. These lipids display an astounding variation in their carbohydrate head groups, suggesting that glycosphingolipids serve specialized functions in recognition processes. It is now realized that they are organized in signalling domains on the cell surface. They are of vital importance as, in their absence, embryonal development is inhibited at an early stage. Remarkably, individual cells can live without glycolipids, perhaps because their survival does not depend on glycosphingolipid-mediated signalling mechanisms. Still, these cells suffer from defects in intracellular membrane transport. Various membrane proteins do not reach their intracellular destination, and, indeed, some intracellular organelles do not properly differentiate to their mature stage. The fact that glycosphingolipids are required for cellular differentiation suggests that there are human diseases resulting from defects in glycosphingolipid synthesis. In addition, the same cellular differentiation processes may be affected by defects in the degradation of glycosphingolipids. At the cellular level, the pathology of glycosphingolipid storage diseases is not completely understood. Cell biological studies on the intracellular fate and function of glycosphingolipids may open new ways to understand and defeat not only lipid storage diseases, but perhaps other diseases that have not been connected to glycosphingolipids so far.