Altered secretion of kidney lysosomal enzymes in the mouse pigment mutants ruby-eye, ruby-eye-2-J, and maroon

Hermansky-Pudlak syndrome: a disease of protein trafficking and organelle function

The Hermansky-Pudlak syndrome (HPS) is a collection of related autosomal recessive disorders which are genetically heterogeneous. There are eight human HPS subtypes, characterized by oculocutaneous albinism and platelet storage disease; prolonged bleeding, congenital neutropenia, pulmonary fibrosis, and granulomatous colitis can also occur. HPS is caused primarily by defects in intracellular protein trafficking that result in the dysfunction of intracellular organelles known as lysosome-related organelles. HPS gene products are all ubiquitously expressed and all associate in various multi-protein complexes, yet HPS has cell type-specific disease expression. Impairment of specialized secretory cells such as melanocytes, platelets, lung alveolar type II epithelial cells and cytotoxic T cells are observed in HPS. This review summarizes recent molecular, biochemical and cell biological analyses together with clinical studies that have led to the correlation of molecular pathology with clinical manifestations and led to insights into such diverse disease processes such as albinism, fibrosis, hemorrhage, and congenital neutropenia.

Normal Lytic Granule Secretion by Cytotoxic T Lymphocytes Deficient in BLOC-1, -2 and -3 and Myosins Va, VIIa and XV

Melanocytes and cells of the immune system share an unusual secretory mechanism which uses the lysosome as a regulated secretory organelle. Recently, a number of the proteins required for these 'secretory lysosomes' to undergo exocytosis have been identified. These include Rab27a, Lyst, Rab geranyl geranyl transferase and the adapter protein complex AP-3. Patients lacking any of these proteins are characterized by the rare combination of albinism and immunodeficiency, revealing roles for these proteins in both melanocyte and immune cell secretion. In order to ask how far the link between albinism and immunodeficiency extends we have examined cytotoxic T-lymphocyte (CTL) secretion from two BLOC-3-deficient patients and seven different mouse models of Hermansky-Pudlak syndrome, all of which display defects in pigmentation and platelet function. We find that CTL function is normal in HPS patients and pale-ear mice deficient in BLOC-3, pallid, muted and sandy mice deficient in BLOC-1, ruby-eye mice deficient in BLOC-2 and buff mice deficient in Vps33a. Similarly, the unconventional myosins, Va, VIIa and XV, which can act as effectors for Rab27a in some cell types, are not required in CTL. These results reveal differences in the protein machinery required for biogenesis and/or secretion of lysosome-related organelles in CTL and melanocytes.

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.

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

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

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

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

The mouse pale ear (ep) mutation is the homologue of human Hermansky-Pudlak syndrome

The recessive mutation at the pale ear (ep) locus on mouse chromosome 19 was found to be the homologue of human Hermansky-Pudlak syndrome (HPS). A positional cloning strategy using yeast artificial chromosomes spanning the HPS locus was used to identify the HPS gene and its murine counterpart. These genes and their predicted proteins are highly conserved at the nucleotide and amino acid levels. Sequence analysis of the mutant ep gene revealed the insertion of an intracisternal A particle element in a protein-coding 3' exon. Here we demonstrate that mice with the ep mutation exhibit abnormalities similar to human HPS patients in melanosomes and platelet-dense granules. These results establish an animal model of HPS and will facilitate biochemical and molecular analyses of the functions of this protein in the membranes of specialized intracellular organelles.

alpha-Glucosidase and N-acetylglucosamine-6-sulphatase are the major mannose-6-phosphate glycoproteins in human urine

Most newly synthesized lysosomal enzymes contain a transient carbohydrate modification, mannose 6-phosphate (Man-6-P), which signals their vesicular transport from the Golgi to the lysosome via Man-6-P receptors (MPRs). We have examined Man-6-P glycoproteins in human urine by using a purified soluble fragment of the soluble cation-independent MPR (sCI-MPR) as a preparative and analytical affinity reagent. In a survey of urine samples from seven healthy subjects, the pattern of Man-6-P glycoproteins detected with iodinated sCI-MPR as a probe in a blotting assay was essentially identical in each, regardless of sex or age. Two bands of approx. 100 and 110 kDa were particularly prominent. Man-6-P glycoproteins in human urine were purified by affinity chromatography on immobilized sCI-MPR. Seven distinct bands revealed by SDS/PAGE and Coomassie Blue staining were subjected to N-terminal sequence analysis. The prominent 100 and 110 kDa Man-6-P glycoproteins were identified as N-acetylglucosamine-6-sulphatase and alpha-glucosidase respectively. This identification was confirmed by molecular mass determinations on the two major bands after deglycosylation. Sequence analysis revealed arylsulphatase A and several previously unidentified proteins as minor species. Man-6-P glycoproteins were also purified on an analytical scale to determine the proportion of a number of lysosomal enzyme activities represented by the mannose-6-phosphorylated forms. The lysosomal enzymes in urine containing the highest proportion of mannose-6-phosphorylated form were beta-mannosidase (82%), hexosaminidase (27%) and alpha-glucosidase (24%). The profiles of Man-6-P glycoproteins detected by blotting in urine and plasma were not similar, suggesting that the urinary species are not derived from the bloodstream.

A fusion pore phenotype in mast cells of the ruby-eye mouse

Using patch-clamp capacitance and amperometric techniques, we have identified an exocytotic phenotype that affects the function of the fusion pore, the molecular structure that connects the lumen of a secretory vesicle with the extracellular environment during exocytosis. Direct observation of individual exocytotic events in mast cells from the ruby-eye mouse (ru/ru) showed a 3-fold increase in the fraction and duration of transient fusion events with respect to wild-type mice. The fraction of the total fusion events that were transient increased from 0.22 +/- 0.02 (wild type) to 0.65 +/- 0.02 (ru/ru), and the average duration of these events increased from 418 +/- 32 ms (wild type) to 1207 +/- 89 ms (ru/ru). We also show that this phenotype can reduce and delay an evoked secretory response by causing the fusion of vesicles that have been previously emptied by repeated cycles of transient fusion. The exocytotic phenotype that we describe here may be a cause of diseases like platelet storage pool deficiency and prolonged bleeding times for which the ruby-eye mouse serves as an animal model. Furthermore, the identification of the gene causing the fusion pore phenotype reported here will illuminate the molecular mechanisms regulating exocytotic fusion.

Correlation between the number of melanosomes, tyrosinase mRNA levels, and tyrosinase activity in cultured murine melanoma cells in response to various melanogenesis regulatory agents

Tyrosinase is the rate limiting enzyme critically associated with melanin synthesis. The melanosomes are specialized membrane-bound organelles within melanocytic cells in which melanin polymers are ultimately deposited. To determine whether tyrosinase correlates with the number of melanosomes, we examined the relationship between tyrosinase activity, tyrosinase mRNA levels, and the number of melanosomes in B16 murine melanoma cells, using melanogenesis regulatory agents. 12-O-Tetradecanoylphorbol-13-acetate (TPA) or linoleic acid decreased tyrosinase activity, while dibutyryl cyclic adenosine monophosphate (dbcAMP) or palmitic acid increased it. The tyrosinase mRNA levels were not always correlated with tyrosinase activity, i.e., TPA down-regulated, dbcAMP upregulated, while linoleic acid or palmitic acid did not alter the message levels, indicating that fatty acid regulation of melanogenesis was due to post-transcriptional events. The number of melanosomes changed when agents which modulate the tyrosinase gene expression were added, since TPA decreased, dbcAMP increased, and linoleic acid or palmitic acid did not alter their number. These results suggest that the number of melanosomes changed in relation to tyrosinase mRNA level but not to tyrosinase activity in response to melanogenesis regulatory agents.

The murine pallid mutation is a platelet storage pool disease associated with the protein 4.2 (pallidin) gene

Pallid is one of 12 independent murine mutations with a prolonged bleeding time that are models for human platelet storage pool deficiencies in which several intracellular organelles are abnormal. We have mapped the murine gene for protein 4.2 (Epb4.2) to chromosome 2 where it co-localizes with pallid. Southern blot analyses suggest that pallid is a mutation in the Epb4.2 gene. Northern blot analyses demonstrate a smaller than normal Epb4.2 transcript in affected pallid tissues, such as kidney and skin. This is the first gene defect to be associated with a platelet storage pool deficiency, and may allow the identification of a novel structure or biological pathway that influences granulogenesis.

Cocoa: a new mouse model for platelet storage pool deficiency

We describe genetic, haematological and biochemical properties of a new mouse pigment mutant, cocoa (coa). Cocoa is a recessive mutation located on the centromeric end of chromosome 3 near the Car-2 locus. The mutation causes increased bleeding time accompanied by symptoms of platelet storage pool deficiency (SPD), including decreased platelet serotonin and decreased visibility of dense granules as analysed by electron microscopy of unfixed platelets. Dense granules were visible in normal numbers when platelets were incubated with the fluorescent dye, mepacrine. The intragranular environment, however, was abnormal as indicated by decreased flashing of mepacrine-loaded dense granules after exposure to ultraviolet light. Unlike the previously described seven mouse pigment mutations with SPD in which pigment granules, platelet dense granules and lysosomes are affected, the cocoa mutant had normal secretion of lysosomal enzymes from kidney proximal tubule cells and platelets. The cocoa mutation thus represents an example of a single gene which simultaneously affects melanosomes and platelet dense granules but probably does not affect lysosomes. The results indicate that melanosomes and platelet dense granules share steps in synthesis and/or processing. Cocoa may be a model for cases of human Hermansky-Pudlak syndrome in which functions of melanosomes and platelet dense granules, but not lysosomes, are involved.

Biochemical and morphological characterization of primary kidney cell cultures from beige mutant mice

Primary kidney cultures from adult beige-J (bgJ/bgJ) mice were selected for epithelial cell growth using D-valine medium. After 2 weeks of attachment and proliferation in vitro, the cells form a confluent or nearly confluent monolayer that retains several phenotypic characteristics of the beige-J mutant. These include large, multilamellar inclusion bodies that are apparently dysmorphic lysosomes, and higher concentrations of neutral glycosphingolipids and dolichols than control cells. beta-Glucuronidase activity, used as a lysosomal enzyme marker, is not elevated in beige-J-cultured kidney cells compared with controls, as it is in the intact kidney. The high levels of beta-glucuronidase activity in both control and mutant cells may mask expression of this difference in vitro. The action of the beige-J mutation in kidney cells is thought to be due to a block in exocytosis that results in the accumulation of abnormal lysosomes and their components. The maintenance of the beige phenotype in vitro indicates that the mutation is not suppressed in primary kidney cell cultures. The expression of the beige phenotype in vitro should be useful for studies concerning the primary lesion of this mutation.

Effects of ammonia on processing and secretion of precursor and mature lysosomal enzyme from macrophages of normal and pale ear mice: evidence for two distinct pathways

Lysosomal enzymes have been shown to be synthesized as microsomal precursors, which are processed to mature enzymes located in lysosomes. We examined the effect of ammonium chloride on the intracellular processing and secretion of two lysosomal enzymes, beta-glucuronidase and beta-galactosidase, in mouse macrophages. This lysosomotropic drug caused extensive secretion of both precursor and mature enzyme forms within a few hours, as documented by pulse radiolabeling and molecular weight analysis. The normal intracellular route for processing and secretion of precursor enzyme was altered in treated cells. A small percentage of each precursor was delivered to the lysosomal organelle slowly. Most precursor forms traversed the Golgi apparatus, underwent further processing of carbohydrate moieties, and were then secreted in a manner similar to secretory proteins. The lag time for secretion of newly synthesized beta-galactosidase precursor was notably longer than that for the beta-glucuronidase precursor. The source of the secreted mature enzyme was the lysosomal organelle. Macrophages from the pale ear mutant were markedly deficient in secretion of mature lysosomal enzyme but secreted precursor forms normally. These results suggest that ammonia-treated macrophages contain two distinct intracellular pathways for secretion of lysosomal enzymes and that a specific block in the release of lysosomal contents occurs in the pale ear mutant.

Discrete visual defects in pearl mutant mice

The mutant mouse pearl, characterized by its hypopigmentation, has a specific functional defect in a sensory system--the retina. The intact pearl mouse has reduced sensitivity in the dark-adapted condition. Normal sensitivity is restored by isolation and superfusion of the retina with bicarbonate-buffered Ringer solution, suggesting that the retinal expression of the pearl mutation depends on a diffusible substance. The pearl phenotype is described as a possible model for human congenital stationary night blindness.