The Beige/Chediak-Higashi syndrome gene encodes a widely expressed cytosolic protein

The lysosomal trafficking regulator "LYST": an 80-year traffic jam

Lysosomes and lysosome related organelles (LROs) are dynamic organelles at the intersection of various pathways involved in maintaining cellular hemostasis and regulating cellular functions. Vesicle trafficking of lysosomes and LROs are critical to maintain their functions. The lysosomal trafficking regulator (LYST) is an elusive protein important for the regulation of membrane dynamics and intracellular trafficking of lysosomes and LROs. Mutations to the LYST gene result in Chédiak-Higashi syndrome, an autosomal recessive immunodeficiency characterized by defective granule exocytosis, cytotoxicity, etc. Despite eight decades passing since its initial discovery, a comprehensive understanding of LYST's function in cellular biology remains unresolved. Accumulating evidence suggests that dysregulation of LYST function also manifests in other disease states. Here, we review the available literature to consolidate available scientific endeavors in relation to LYST and discuss its relevance for immunomodulatory therapies, regenerative medicine and cancer applications.

Spectrum of LYST mutations in Chediak-Higashi syndrome: a report of novel variants and a comprehensive review of the literature

INTRODUCTION

Chediak-Higashi syndrome (CHS) is a rare autosomal recessive disorder characterised by partial oculocutaneous albinism, a bleeding diathesis, immunological dysfunction and neurological impairment. Bi-allelic loss-of-function variants in LYST cause CHS. LYST encodes the lysosomal trafficking regulator, a highly conserved 429 kDa cytoplasmic protein with an unknown function.

METHODS

To further our understanding of the pathogenesis of CHS, we conducted clinical evaluations on individuals with CHS enrolled in our natural history study. Using genomic DNA Sanger sequencing, we identified novel pathogenic LYST variants. Additionally, we performed an extensive literature review to curate reported LYST variants and classified these novel and reported variants according to the American College of Medical Genetics/Association for Molecular Pathology variant interpretation guidelines.

RESULTS

Our investigation unveiled 11 novel pathogenic LYST variants in eight patients with a clinical diagnosis of CHS, substantiated by the presence of pathognomonic giant intracellular granules. From these novel variants, together with a comprehensive review of the literature, we compiled a total of 147 variants in LYST, including 61 frameshift variants (41%), 44 nonsense variants (30%), 23 missense variants (16%), 13 splice site variants or small genomic deletions for which the coding effect is unknown (9%), 5 in-frame variants (3%) and 1 start-loss variant (1%). Notably, a genotype-phenotype correlation emerged, whereby individuals harbouring at least one missense or in-frame variant generally resulted in milder disease, while those with two nonsense or frameshift variants generally had more severe disease.

CONCLUSION

The identification of novel pathogenic LYST variants and improvements in variant classification will provide earlier diagnoses and improved care to individuals with CHS.

LYST deficiency impairs autophagic lysosome reformation in neurons and alters lysosome number and size

Chediak-Higashi syndrome (CHS) is a rare, autosomal recessive disorder caused by biallelic mutations in the lysosomal trafficking regulator (LYST) gene. Even though enlarged lysosomes and/or lysosome-related organelles (LROs) are the typical cellular hallmarks of CHS, they have not been investigated in human neuronal models. Moreover, how and why the loss of LYST function causes a lysosome phenotype in cells has not been elucidated. We report that the LYST-deficient human neuronal model exhibits lysosome depletion accompanied by hyperelongated tubules extruding from enlarged autolysosomes. These results have also been recapitulated in neurons differentiated from CHS patients' induced pluripotent stem cells (iPSCs), validating our model system. We propose that LYST ensures the correct fission/scission of the autolysosome tubules during autophagic lysosome reformation (ALR), a crucial process to restore the number of free lysosomes after autophagy. We further demonstrate that LYST is recruited to the lysosome membrane, likely to facilitate the fission of autolysosome tubules. Together, our results highlight the key role of LYST in maintaining lysosomal homeostasis following autophagy and suggest that ALR dysregulation is likely associated with the neurodegenerative CHS phenotype.

cDNA sequencing increases the molecular diagnostic yield in Chediak-Higashi syndrome

Introduction: Chediak-Higashi syndrome (CHS) is rare autosomal recessive disorder caused by bi-allelic variants in the Lysosomal Trafficking Regulator (LYST) gene. Diagnosis is established by the detection of pathogenic variants in LYST in combination with clinical evidence of disease. Conventional molecular genetic testing of LYST by genomic DNA (gDNA) Sanger sequencing detects the majority of pathogenic variants, but some remain undetected for several individuals clinically diagnosed with CHS. In this study, cDNA Sanger sequencing was pursued as a complementary method to identify variant alleles that are undetected by gDNA Sanger sequencing and to increase molecular diagnostic yield. Methods: Six unrelated individuals with CHS were clinically evaluated and included in this study. gDNA Sanger sequencing and cDNA Sanger sequencing were performed to identify pathogenic LYST variants. Results: Ten novel LYST alleles were identified, including eight nonsense or frameshift variants and two in-frame deletions. Six of these were identified by conventional gDNA Sanger sequencing; cDNA Sanger sequencing was required to identify the remaining variant alleles. Conclusion: By utilizing cDNA sequencing as a complementary technique to identify LYST variants, a complete molecular diagnosis was obtained for all six CHS patients. In this small CHS cohort, the molecular diagnostic yield was increased, and canonical splice site variants identified from gDNA Sanger sequencing were validated by cDNA sequencing. The identification of novel LYST alleles will aid in diagnosing patients and these molecular diagnoses will also lead to genetic counseling, access to services and treatments and clinical trials in the future.

The yeast LYST homolog Bph1 is a Rab5 effector and prevents Atg8 lipidation at endosomes

Lysosomes mediate degradation of macromolecules to their precursors for their cellular recycling. Additionally, lysosome-related organelles mediate cell type-specific functions. The Chédiak-Higashi syndrome is an autosomal, recessive disease, in which loss of the protein LYST causes defects in lysosomes and lysosome-related organelles. The molecular function of LYST, however, is largely unknown. Here, we dissected the function of the yeast LYST homolog, Bph1. We show that Bph1 is an endosomal protein, and an effector of the minor Rab5 isoform Ypt52. Strikingly, the bph1▵ mutant has lipidated Atg8 on their endosomes, which is sorted via late endosomes into the vacuole lumen under non-autophagy inducing conditions. In agreement, proteomics of bph1▵ vacuoles reveal an accumulation of Atg8, reduced flux via selective autophagy, and defective endocytosis. Additionally, bph1▵ cells have reduced autophagic flux under starvation conditions. Our observations suggest that Bph1 is a novel Rab5 effector that maintains endosomal functioning. When lost, Atg8 is lipidated at endosomes even during normal growth and ends up in the vacuole lumen. Thus, our results contribute to the understanding of the role of LYST-related proteins and associated diseases.

Deficiency in Lyst function leads to accumulation of secreted proteases and reduced retinal adhesion

Chediak–Higashi syndrome, caused by mutations in the Lysosome Trafficking Regulator (Lyst) gene, is a recessive hypopigmentation disorder characterized by albinism, neuropathies, neurodegeneration, and defective immune responses, with enlargement of lysosomes and lysosome-related organelles. Although recent studies have suggested that Lyst mutations impair the regulation of sizes of lysosome and lysosome-related organelle, the underlying pathogenic mechanism of Chediak–Higashi syndrome is still unclear. Here we show striking evidence that deficiency in LYST protein function leads to accumulation of photoreceptor outer segment phagosomes in retinal pigment epithelial cells, and reduces adhesion between photoreceptor outer segment and retinal pigment epithelial cells in a mouse model of Chediak–Higashi syndrome. In addition, we observe elevated levels of cathepsins, matrix metallopeptidase (MMP) 3 and oxidative stress markers in the retinal pigment epithelium of Lyst mutants. Previous reports showed that impaired degradation of photoreceptor outer segment phagosomes causes elevated oxidative stress, which could consequently lead to increases of cysteine cathepsins and MMPs in the extracellular matrix. Taken together, we conclude that the loss of LYST function causes accumulation of phagosomes in the retinal pigment epithelium and elevation of several extracellular matrix-remodeling proteases through oxidative stress, which may, in turn, reduce retinal adhesion. Our work reveals previously unreported pathogenic events in the retinal pigment epithelium caused by Lyst deficiency. The same pathogenic events may be conserved in other professional phagocytic cells, such as macrophages in the immune system, contributing to overall Chediak–Higashi syndrome pathology.

The lysosomal trafficking regulator is necessary for normal wound healing

Non-healing wounds are a major threat to public health throughout the United States. Tissue healing is complex multifactorial process that requires synchronicity of several cell types. Endolysosomal trafficking, which contributes to various cell functions from protein degradation to plasma membrane repair, is an understudied process in the context of wound healing. The lysosomal trafficking regulator protein (LYST) is an essential protein of the endolysosomal system through an indeterminate mechanism. In this study, we examine the impact of impaired LYST function both in vitro with primary LYST mutant fibroblasts as well as in vivo with an excisional wound model. The wound model shows that LYST mutant mice have impaired wound healing in the form of delayed epithelialization and collagen deposition, independent of macrophage infiltration and polarisation. We show that LYST mutation confers a deficit in MCP-1, IGF-1, and IGFBP-2 secretion in beige fibroblasts, which are critical factors in normal wound healing. Identifying the mechanism of LYST function is important for understanding normal wound biology, which may facilitate the development of strategies to address problem wound healing.

Reactive oxygen species prevent lysosome coalescence during PIKfyve inhibition

Lysosomes are terminal, degradative organelles of the endosomal pathway that undergo repeated fusion-fission cycles with themselves, endosomes, phagosomes, and autophagosomes. Lysosome number and size depends on balanced fusion and fission rates. Thus, conditions that favour fusion over fission can reduce lysosome numbers while enlarging their size. Conversely, favouring fission over fusion may cause lysosome fragmentation and increase their numbers. PIKfyve is a phosphoinositide kinase that generates phosphatidylinositol-3,5-bisphosphate to modulate lysosomal functions. PIKfyve inhibition causes an increase in lysosome size and reduction in lysosome number, consistent with lysosome coalescence. This is thought to proceed through reduced lysosome reformation and/or fission after fusion with endosomes or other lysosomes. Previously, we observed that photo-damage during live-cell imaging prevented lysosome coalescence during PIKfyve inhibition. Thus, we postulated that lysosome fusion and/or fission dynamics are affected by reactive oxygen species (ROS). Here, we show that ROS generated by various independent mechanisms all impaired lysosome coalescence during PIKfyve inhibition and promoted lysosome fragmentation during PIKfyve re-activation. However, depending on the ROS species or mode of production, lysosome dynamics were affected distinctly. H2O2 impaired lysosome motility and reduced lysosome fusion with phagosomes, suggesting that H2O2 reduces lysosome fusogenecity. In comparison, inhibitors of oxidative phosphorylation, thiol groups, glutathione, or thioredoxin, did not impair lysosome motility but instead promoted clearance of actin puncta on lysosomes formed during PIKfyve inhibition. Additionally, actin depolymerizing agents prevented lysosome coalescence during PIKfyve inhibition. Thus, we discovered that ROS can generally prevent lysosome coalescence during PIKfyve inhibition using distinct mechanisms depending on the type of ROS.

Mauve/LYST limits fusion of lysosome-related organelles and promotes centrosomal recruitment of microtubule nucleating proteins

Lysosome-related organelles (LROs) are endosomal compartments carrying tissue-specific proteins, which become enlarged in Chediak-Higashi syndrome (CHS) due to mutations in LYST. Here, we show that Drosophila Mauve, a counterpart of LYST, suppresses vesicle fusion events with lipid droplets (LDs) during the formation of yolk granules (YGs), the LROs of the syncytial embryo, and opposes Rab5, which promotes fusion. Mauve localizes on YGs and at spindle poles, and it co-immunoprecipitates with the LDs' component and microtubule-associated protein Minispindles/Ch-TOG. Minispindles levels are increased at the enlarged YGs and diminished around centrosomes in mauve-derived mutant embryos. This leads to decreased microtubule nucleation from centrosomes, a defect that can be rescued by dominant-negative Rab5. Together, this reveals an unanticipated link between endosomal vesicles and centrosomes. These findings establish Mauve/LYST's role in regulating LRO formation and centrosome behavior, a role that could account for the enlarged LROs and centrosome positioning defects at the immune synapse of CHS patients.

Chediak-Higashi syndrome: a review of the past, present, and future

Since the initial description of Chediak-Higashi syndrome (CHS), over 75 years ago, several studies have been conducted to underscore the role of the lysosomal trafficking regulator (LYST) gene in the pathogenesis of disease. CHS is a rare autosomal recessive disorder, which is caused by biallelic mutations in the highly conserved LYST gene. The disease is characterized by partial oculocutaneous albinism, prolonged bleeding, immune and neurologic dysfunction, and risk for the development of hemophagocytic lympohistiocytosis (HLH). The presence of giant secretory granules in leukocytes is the classical diagnostic feature, which distinguishes CHS from closely related Griscelli and Hermansky-Pudlak syndromes. While the exact mechanism of the formation of the giant granules in CHS patients is not understood, dysregulation of LYST function in regulating lysosomal biogenesis has been proposed to play a role. In this review, we discuss the clinical characteristics of the disease and highlight the functional consequences of enlarged lysosomes and lysosome-related organelles (LROs) in CHS.

Primary Immunodeficiency in the NICU

Primary immunodeficiency disorders (PIDs) are genetic diseases that lead to increased susceptibility to infection. Hundreds of PIDs have now been described, but a select subset commonly presents in the neonatal period. Neonates, especially premature newborns, have relative immune immaturity that makes it challenging to differentiate PIDs from intrinsic immaturity. Nonetheless, early identification and appropriate management of PIDs are critical, and the neonatal clinician should be familiar with a range of PIDs and their presentations. The neonatal clinician should also be aware of the importance of consulting with an immunologist when a PID is suspected. The role of newborn screening for severe combined immunodeficiency, as well as the initial steps of laboratory evaluation for a PID should be familiar to those caring for neonates. Finally, it is important for providers to be familiar with the initial management steps that can be taken to reduce the risk of infection in affected patients.

An actin cytoskeletal barrier inhibits lytic granule release from natural killer cells in patients with Chediak-Higashi syndrome

Background

Chediak-Higashi syndrome (CHS) is a rare disorder caused by biallelic mutations in the lysosomal trafficking regulator gene (LYST), resulting in formation of giant lysosomes or lysosome-related organelles in several cell types. The disease is characterized by immunodeficiency and a fatal hemophagocytic lymphohistiocytosis caused by impaired function of cytotoxic lymphocytes, including natural killer (NK) cells.

Objective

We sought to determine the underlying biochemical cause of the impaired cytotoxicity of NK cells in patients with CHS.

Methods

We generated a human cell model of CHS using Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) technology. We used a combination of classical techniques to evaluate lysosomal function and cell activity in the model system and super-resolution microscopy to visualize F-actin and lytic granules in normal and LYST-deficient NK cells.

Results

Loss of LYST function in a human NK cell line, NK92mi, resulted in inhibition of NK cell cytotoxicity and reproduced other aspects of the CHS cellular phenotype, including the presence of significantly enlarged lytic granules with defective exocytosis and impaired integrity of endolysosomal compartments. The large granules had an acidic pH and normal activity of lysosomal enzymes and were positive for the proteins essential for lytic granule exocytosis. Visualization of the actin meshwork openings at the immunologic synapse revealed that the cortical actin acts as a barrier for secretion of such large granules at the cell-cell contact site. Decreasing the cortical actin density at the immunologic synapse or decreasing the lytic granule size restored the ability of LYST-deficient NK cells to degranulate and kill target cells.

Conclusion

The cortical actin and granule size play significant roles in NK cell cytotoxic function. We present evidence that the periodicity of subsynaptic actin is an important factor limiting the release of large lytic granules from NK cells from patients with CHS and could be a novel target for pharmaceutical intervention.

Differences in Granule Morphology yet Equally Impaired Exocytosis among Cytotoxic T Cells and NK Cells from Chediak-Higashi Syndrome Patients

Chediak–Higashi syndrome (CHS) is caused by autosomal recessive mutations in LYST, resulting in enlarged lysosomal compartments in multiple cell types. CHS patients display oculocutaneous albinism and may develop life-threatening hemophagocytic lymphohistiocytosis (HLH). While NK cell-mediated cytotoxicity has been reported to be uniformly defective, variable defects in T cell-mediated cytotoxicity has been observed. The latter has been linked to the degree of HLH susceptibility. Since the discrepancies in NK cell- and T cell-mediated cellular cytotoxicity might result from differences in regulation of cytotoxic granule release, we here evaluated perforin-containing secretory lysosome size and number in freshly isolated lymphocytes from CHS patients and furthermore compared their exocytic capacities. Whereas NK cells from CHS patients generally contained a single, gigantic perforin-containing granule, cytotoxic T cells predominantly contained several smaller granules. Nonetheless, in a cohort of 21 CHS patients, cytotoxic T cell and NK cell granule exocytosis were similarly impaired upon activating receptor stimulation. Mechanistically, polarization of cytotoxic granules was defective in cytotoxic lymphocytes from CHS patients, with EEA1, a marker of early endosomes, mislocalizing to lysosomal structures. The results leads to the conclusion that lysosome enlargement corresponds to loss of distinct organelle identity in the endocytic pathway, which on a subcellular level more adversely affects NK cells than T cells. Hence, vesicular size or numbers do not per se dictate the impairment of lysosomal exocytosis in the two cell types studied.

Physical, Functional and Genetic Interactions between the BEACH Domain Protein SPIRRIG and LIP5 and SKD1 and Its Role in Endosomal Trafficking to the Vacuole in Arabidopsis

Beige and Chediak Higashi (BEACH) domain-containing proteins (BDCPs) are facilitators of membrane-dependent cellular processes in eukaryotes. Mutations in BDCPs cause malfunctions of endosomal compartments in various cell types. Recently, the molecular analysis of the BDCP homolog gene SPIRRIG (SPI) has revealed a molecular function in P-bodies and the regulation of RNA stability. We therefore aimed to analyze, whether SPI has also a role in membrane-dependent processes. In this study, we show that SPI physically interacts with endosomal sorting complex required for transport associated ATPase Suppressor of K⁺-transport growth defect1 (SKD1) and its positive regulator, LYST Interacting Protein 5 (LIP5) and report genetic interactions between SPI and SKD1 and LIP5. We further show that the endosomal transport route of soluble proteins to the lytic vacuole is disturbed in spi lip5 double mutants but not in the single mutants. These vacuolar transport defects were suppressed by additional expression of SKD1. Our results indicate that the BEACH domain protein SPI has in addition to a role in P-bodies a function in endosomal transport routes.

Large granules in the peripheral blood smear and bone marrow aspirate of a 3-year-old male with lymphadenopathy and fever

A 3-year-old male with oculocutaneous albinism presented with lymphadenopathy and fever. Serological testing revealed Epstein-Barr virus (EBV)-specific immunoglobulin M (IgM) and a diagnosis of infectious mononucleosis was made. A complete blood count and peripheral blood smear demonstrated mild anemia, thrombocytopenia, and neutropenia with leukocytes that contained large azurophilic and eosinophilic granules. Bone marrow examination demonstrated increased hemophagocytic histiocytes along with granulocytes that contained large eosinophilic granules. In addition to hemophagocytic lymphohistiocytosis, presumably due to acute EBV infection, the patient was diagnosed with Chediak-Higashi syndrome based on the pathognomonic granules within peripheral leukocytes and precursors. The differential diagnosis of a young patient with oculocutaneous albinism presenting with an acute viral infection includes a relatively narrow range of genetic syndromes based solely on the history of albinism. This case demonstrates the application of clinical laboratory data to presumptively diagnose Chediak-Higashi syndrome in the midst of a presentation of hemophagocytic lymphohistiocytosis secondary to acute EBV infection.

Lysosomal Trafficking Regulator (LYST)

Regulation of vesicle trafficking to lysosomes and lysosome-related organelles (LROs) as well as regulation of the size of these organelles are critical to maintain their functions. Disruption of the lysosomal trafficking regulator (LYST) results in Chediak-Higashi syndrome (CHS), a rare autosomal recessive disorder characterized by oculocutaneous albinism, prolonged bleeding, severe immunodeficiency, recurrent bacterial infection, neurologic dysfunction and hemophagocytic lympohistiocytosis (HLH). The classic diagnostic feature of the syndrome is enlarged LROs in all cell types, including lysosomes, melanosomes, cytolytic granules and platelet dense bodies. The most striking CHS ocular pathology observed is an enlargement of melanosomes in the retinal pigment epithelium (RPE), which leads to aberrant distribution of eye pigmentation, and results in photophobia and decreased visual acuity. Understanding the molecular function of LYST and identification of its interacting partners may provide therapeutic targets for CHS and other diseases associated with the regulation of LRO size and/or vesicle trafficking, such as asthma, urticaria and Leishmania amazonensis infections.

Natural killer cell biology illuminated by primary immunodeficiency syndromes in humans

Natural killer (NK) cells are innate immune cytotoxic effector cells well known for their role in antiviral immunity and tumor immunosurveillance. In parts, this knowledge stems from rare inherited immunodeficiency disorders in humans that abrogate NK cell function leading to immune impairments, most notably associated with a high susceptibility to viral infections. Phenotypically, these disorders range from deficiencies selectively affecting NK cells to complex general immune defects that affect NK cells but also other immune cell subsets. Moreover, deficiencies may be associated with reduced NK cell numbers or rather impair specific NK cell effector functions. In recent years, genetic defects underlying the various NK cell deficiencies have been uncovered and have triggered investigative efforts to decipher the molecular mechanisms underlying these disorders. Here we review the associations between inherited human diseases and NK cell development as well as function, with a particular focus on defects in NK cell exocytosis and cytotoxicity. Furthermore we outline how reports of diverse genetic defects have shaped our understanding of NK cell biology.

Chediak-Higashi syndrome: Lysosomal trafficking regulator domains regulate exocytosis of lytic granules but not cytokine secretion by natural killer cells

BACKGROUND

Mutations in lysosomal trafficking regulator (LYST) cause Chediak-Higashi syndrome (CHS), a rare immunodeficiency with impaired cytotoxic lymphocyte function, mainly that of natural killer (NK) cells. Our understanding of NK cell function deficiency in patients with CHS and how LYST regulates lytic granule exocytosis is very limited.

OBJECTIVE

We sought to delineate cellular defects associated with LYST mutations responsible for the impaired NK cell function seen in patients with CHS.

METHODS

We analyzed NK cells from patients with CHS with missense mutations in the LYST ARM/HEAT (armadillo/huntingtin, elongation factor 3, protein phosphatase 2A, and the yeast kinase TOR1) or BEACH (beige and Chediak-Higashi) domains.

RESULTS

NK cells from patients with CHS displayed severely reduced cytotoxicity. Mutations in the ARM/HEAT domain led to a reduced number of perforin-containing granules, which were significantly increased in size but able to polarize to the immunologic synapse; however, they were unable to properly fuse with the plasma membrane. Mutations in the BEACH domain resulted in formation of normal or slightly enlarged granules that had markedly impaired polarization to the IS but could be exocytosed on reaching the immunologic synapse. Perforin-containing granules in NK cells from patients with CHS did not acquire certain lysosomal markers (lysosome-associated membrane protein 1/2) but were positive for markers of transport vesicles (cation-independent mannose 6-phosphate receptor), late endosomes (Ras-associated binding protein 27a), and, to some extent, early endosomes (early endosome antigen 1), indicating a lack of integrity in the endolysosomal compartments. NK cells from patients with CHS had normal cytokine compartments and cytokine secretion.

CONCLUSION

LYST is involved in regulation of multiple aspects of NK cell lytic activity, ranging from governance of lytic granule size to control of their polarization and exocytosis, as well as regulation of endolysosomal compartment identity. LYST functions in the regulated exocytosis but not in the constitutive secretion pathway.

Optic neuropathy in late-onset neurodegenerative Chédiak-Higashi syndrome

BACKGROUND

The classic form of Chédiak-Higashi syndrome (CHS), an autosomal recessive disorder of lysosomal trafficking with childhood onset caused by mutations in ITALIC! LYST, is typified ophthalmologically by ocular albinism with vision loss attributed to foveal hypoplasia or nystagmus. Optic nerve involvement and ophthalmological manifestations of the late-onset neurodegenerative form of CHS are rarely reported and poorly detailed.

METHODS

Case series detailing ophthalmological and neurological findings in three adult siblings with the late-onset form of CHS.

RESULTS

All three affected siblings lacked features of ocular albinism and demonstrated significant optic nerve involvement as evidenced by loss of colour and contrast vision, central visual field loss, optic nerve pallor, retinal nerve fibre layer thinning by optical coherence tomography (OCT) and abnormal visual evoked potential, with severity corresponding linearly to age of the sibling and severity of neurological disease. Further, unusual prominence of a 'third line' on macular OCT that may be due to abnormal melanosomes was seen in all three siblings and in their father. Neurological involvement included parkinsonism, cerebellar ataxia and spastic paraparesis.

CONCLUSIONS

This report expands the ophthalmological phenotype of the late-onset neurodegenerative form of CHS to include optic neuropathy with progressive vision loss, even in the absence of ocular albinism, and abnormal prominence of the interdigitation zone between cone outer segment tips and apical processes of retinal pigment epithelium cells on macular OCT.

Reactivation of Lysosomal Ca2+ Efflux Rescues Abnormal Lysosomal Storage in FIG4-Deficient Cells

Loss of function of FIG4 leads to Charcot-Marie-Tooth disease Type 4J, Yunis-Varon syndrome, or an epilepsy syndrome. FIG4 is a phosphatase with its catalytic specificity toward 5'-phosphate of phosphatidylinositol-3,5-diphosphate (PI3,5P2). However, the loss of FIG4 decreases PI3,5P2 levels likely due to FIG4's dominant effect in scaffolding a PI3,5P2 synthetic protein complex. At the cellular level, all these diseases share similar pathology with abnormal lysosomal storage and neuronal degeneration. Mice with no FIG4 expression (Fig4(-/-)) recapitulate the pathology in humans with FIG4 deficiency. Using a flow cytometry technique that rapidly quantifies lysosome sizes, we detected an impaired lysosomal fission, but normal fusion, in Fig4(-/-) cells. The fission defect was associated with a robust increase of intralysosomal Ca(2+) in Fig4(-/-) cells, including FIG4-deficient neurons. This finding was consistent with a suppressed Ca(2+) efflux of lysosomes because the endogenous ligand of lysosomal Ca(2+) channel TRPML1 is PI3,5P2 that is deficient in Fig4(-/-) cells. We reactivated the TRPML1 channels by application of TRPML1 synthetic ligand, ML-SA1. This treatment reduced the intralysosomal Ca(2+) level and rescued abnormal lysosomal storage in Fig4(-/-) culture cells and ex vivo DRGs. Furthermore, we found that the suppressed Ca(2+) efflux in Fig4(-/-) culture cells and Fig4(-/-) mouse brains profoundly downregulated the expression/activity of dynamin-1, a GTPase known to scissor organelle membranes during fission. This downregulation made dynamin-1 unavailable for lysosomal fission. Together, our study revealed a novel mechanism explaining abnormal lysosomal storage in FIG4 deficiency. Synthetic ligands of the TRPML1 may become a potential therapy against diseases with FIG4 deficiency.

LYST controls the biogenesis of the endosomal compartment required for secretory lysosome function

Chediak-Higashi syndrome (CHS) is caused by mutations in the gene encoding LYST protein, the function of which remains poorly understood. Prominent features of CHS include defective secretory lysosome exocytosis and the presence of enlarged, lysosome-like organelles in several cell types. In order to get further insight into the role of LYST in the biogenesis and exocytosis of cytotoxic granules, we analyzed cytotoxic T lymphocytes (CTLs) from patients with CHS. Using confocal microscopy and correlative light electron microscopy, we showed that the enlarged organelle in CTLs is a hybrid compartment that contains proteins components from recycling-late endosomes and lysosomes. Enlargement of cytotoxic granules results from the progressive clustering and then fusion of normal-sized endolysosomal organelles. At the immunological synapse (IS) in CHS CTLs, cytotoxic granules have limited motility and appear docked while nevertheless unable to degranulate. By increasing the expression of effectors of lytic granule exocytosis, such as Munc13-4, Rab27a and Slp3, in CHS CTLs, we were able to restore the dynamics and the secretory ability of cytotoxic granules at the IS. Our results indicate that LYST is involved in the trafficking of the effectors involved in exocytosis required for the terminal maturation of perforin-containing vesicles into secretory cytotoxic granules.

Dictyostelium LvsB has a regulatory role in endosomal vesicle fusion

Defects in human lysosomal-trafficking regulator (Lyst) are associated with the lysosomal disorder Chediak-Higashi syndrome. The absence of Lyst results in the formation of enlarged lysosome-related compartments, but the mechanism for how these compartments arise is not well established. Two opposing models have been proposed to explain Lyst function. The fission model describes Lyst as a positive regulator of fission from lysosomal compartments, whereas the fusion model identifies Lyst as a negative regulator of fusion between lysosomal vesicles. Here, we used assays that can distinguish between defects in vesicle fusion versus fission. We compared the phenotype of Dictyostelium discoideum cells defective in LvsB, the ortholog of Lyst, with that of two known fission defect mutants (μ3- and WASH-null mutants). We found that the temporal localization characteristics of the post-lysosomal marker vacuolin, as well as vesicular acidity and the fusion dynamics of LvsB-null cells are distinct from those of both μ3- and WASH-null fission defect mutants. These distinctions are predicted by the fusion defect model and implicate LvsB as a negative regulator of vesicle fusion.

Enhanced diacylglycerol production by phospholipase D activation is responsible for abnormal increase in concanavalin A cap formation in polymorphonuclear leukocytes from Chediak-Higashi syndrome (beige) mice

We previously reported that enhanced ceramide production induces calpain-mediated proteolysis of protein kinase C (PKC) in leukocytes from Chediak-Higashi syndrome (CHS). In the present study, we demonstrated that phospholipase D (PLD) inhibitors ameliorated abnormal increases in concanavalin A (Con A) cap formation in polymorphonuclear leukocytes (PMNs) from beige mouse, an animal model of CHS. PLD activity in PMNs from beige mice enhanced at 30 to 60s after Con A stimulation. In Con A-stimulated beige PMNs, both neutral sphingomyelinase (N-SMase) and acidic sphingomyelinase (A-SMase) activities enhanced, and ceramide levels are also increased. We found that ceramide levels were reversed by the treatment of beige PMNs with propranolol which inhibits phosphatidic acid phosphohydrolase. In addition, we showed that diacylgycerol (DAG) analogs enhance both N-SMase and A-SMase activities in PMNs from normal mice. We subsequently examined the association of CHS1 with PLD, and showed that expression of a truncated mutant of CHS1 in 293T cells induced abnormally rapid activation of PLD after phorbol ester stimulation. Moreover, we showed that specific inhibitors of 14-3-3 proteins, which interact with CHS1/LYST and bind PKC, did not affect abnormal increases in Con A cap formation in beige PMNs. These results suggest that the enhanced DAG production via the PLD pathway is associated with abnormal increases in Con A cap formation in beige PMNs, and that CHS1 may be involved in the regulation of PLD activity.

Clinical, laboratory and molecular signs of immunodeficiency in patients with partial oculo-cutaneous albinism

Hypopigmentation disorders that are associated with immunodeficiency feature both partial albinism of hair, skin and eyes together with leukocyte defects. These disorders include Chediak Higashi (CHS), Griscelli (GS), Hermansky-Pudlak (HPS) and MAPBP-interacting protein deficiency syndromes. These are heterogeneous autosomal recessive conditions in which the causal genes encode proteins with specific roles in the biogenesis, function and trafficking of secretory lysosomes. In certain specialized cells, these organelles serve as a storage compartment. Impaired secretion of specific effector proteins from that intracellular compartment affects biological activities. In particular, these intracellular granules are essential constituents of melanocytes, platelets, granulocytes, cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. Thus, abnormalities affect pigmentation, primary hemostasis, blood cell counts and lymphocyte cytotoxic activity against microbial pathogens. Among eight genetically distinct types of HPS, only type 2 is characterized by immunodeficiency. Recently, a new subtype, HPS9, was defined in patients presenting with immunodeficiency and oculocutaneous albinism, associated with mutations in the pallidin-encoding gene, PLDN.Hypopigmentation together with recurrent childhood bacterial or viral infections suggests syndromic albinism. T and NK cell cytotoxicity are generally impaired in patients with these disorders. Specific clinical and biochemical phenotypes can allow differential diagnoses among these disorders before molecular testing. Ocular symptoms, including nystagmus, that are usually evident at birth, are common in patients with HPS2 or CHS. Albinism with short stature is unique to MAPBP-interacting protein (MAPBPIP) deficiency, while hemophagocytic lymphohistiocytosis (HLH) mainly suggests a diagnosis of CHS or GS type 2 (GS2). Neurological disease is a long-term complication of CHS, but is uncommon in other syndromic albinism. Chronic neutropenia is a feature of HPS2 and MAPBPIP-deficiency syndrome, whereas it is usually transient in CHS and GS2. In every patient, an accurate diagnosis is required for prompt and appropriate treatment, particularly in patients who develop HLH or in whom bone marrow transplant is required. This review describes the molecular and pathogenetic mechanisms of these diseases, focusing on clinical and biochemical aspects that allow early differential diagnosis.

Drosophila mauve mutants reveal a role of LYST homologs late in the maturation of phagosomes and autophagosomes

Chediak-Higashi syndrome (CHS) is a lethal disease caused by mutations that inactivate the lysosomal trafficking regulator protein (LYST). Patients suffer from diverse symptoms including oculocutaneous albinism, recurrent infections, neutropenia and progressive neurodegeneration. These defects have been traced back to over-sized lysosomes and lysosome-related organelles (LROs) in different cell types. Here, we explore mutants in the Drosophila mauve gene as a new model system for CHS. The mauve gene (CG42863) encodes a large BEACH domain protein of 3535 amino acids similar to LYST. This reflects a functional homology between these proteins as mauve mutants also display enlarged LROs, such as pigment granules. This Drosophila model also replicates the enhanced susceptibility to infections and we show a defect in the cellular immune response. Early stages of phagocytosis proceed normally in mauve mutant hemocytes but, unlike in wild type, late phagosomes fuse and generate large vacuoles containing many bacteria. Autophagy is similarly affected in mauve fat bodies as starvation-induced autophagosomes grow beyond their normal size. Together these data suggest a model in which Mauve functions to restrict homotypic fusion of different pre-lysosomal organelles and LROs.

The enlarged lysosomes in beige j cells result from decreased lysosome fission and not increased lysosome fusion

Chediak-Higashi syndrome is an autosomal recessive disorder that affects vesicle morphology. The Chs1/Lyst protein is a member of the BEige And CHediak family of proteins. The absence of Chs1/Lyst gives rise to enlarged lysosomes. Lysosome size is regulated by a balance between vesicle fusion and fission and can be reversibly altered by acidifying the cytoplasm using Acetate Ringer's or by incubating with the drug vacuolin-1. We took advantage of these procedures to determine rates of lysosome fusion and fission in the presence or absence of Chs1/Lyst. Here, we show by microscopy, flow cytometry and in vitro fusion that the absence of the Chs1/Lyst protein does not increase the rate of lysosome fusion. Rather, our data indicate that loss of this protein decreases the rate of lysosome fission. We further show that overexpression of the Chs1/Lyst protein gives rise to a faster rate of lysosome fission. These results indicate that Chs1/Lyst regulates lysosome size by affecting fission.

A novel single point mutation of the LYST gene in two siblings with different phenotypic features of Chediak Higashi syndrome

Chediak Higashi syndrome (CHS) is an autosomal-recessive disorder characterized by oculocutaneous albinism, recurrent infections and a progressive primary neurological disease. Here, we describe two siblings with CHS due to a novel homozygous R1836X mutation in the LYST gene associated with loss of NK cell degranulation and cytotoxicity. While one sibling was born with fair skin and hair and died of hemophagocytic lymphohistiocytosis (HLH) at 5 months of age, the other sibling had dark black hair and skin and developed HLH at the age of 4 years.

LAMP-2-deficient human B cells exhibit altered MHC class II presentation of exogenous antigens

Major histocompatibility complex (MHC) class II molecules present antigenic peptides derived from engulfed exogenous proteins to CD4(+) T cells. Exogenous antigens are processed in mature endosomes and lysosomes where acidic proteases reside and peptide-binding to class II alleles is favoured. Hence, maintenance of the microenvironment within these organelles is probably central to efficient MHC class II-mediated antigen presentation. Lysosome-associated membrane proteins such as LAMP-2 reside in mature endosomes and lysosomes, yet their role in exogenous antigen presentation pathways remains untested. In this study, human B cells lacking LAMP-2 were examined for changes in MHC class II-restricted antigen presentation. MHC class II presentation of exogenous antigen and peptides to CD4(+) T cells was impaired in the LAMP-2-deficient B cells. Peptide-binding to MHC class II on LAMP-2-deficient B cells was reduced at physiological pH compared with wild-type cells. However, peptide-binding and class II-restricted antigen presentation were restored by incubation of LAMP-2-negative B cells at acidic pH, suggesting that efficient loading of exogenous epitopes by MHC class II molecules is dependent upon LAMP-2 expression in B cells. Interestingly, class II presentation of an epitope derived from an endogenous transmembrane protein was detected using LAMP-2-deficient B cells. Consequently, LAMP-2 may control the repertoire of peptides displayed by MHC class II molecules on B cells and influence the balance between endogenous and exogenous antigen presentation.

Novel Heterogenous CHS1 Mutations Identified in Five Japanese Patients with Chediak-Higashi Syndrome

Chediak-Higashi syndrome (CHS) is a rare, autosomal recessive disorder characterized by oculocutaneous albinism, recurrent bacterial infections and progressive neurological dysfunction. We demonstrate novel heterogenous mutations of CHS1, the responsive gene of CHS, identified in five Japanese patients with CHS. Patients 1, 2, and 3 were siblings, and they had albinism of the skin and hair. They all had a heterogenous two-base deletion (c.5541-5542 del AA, p.Q1847fsX1850) in exon 18. Patient 4 had a heterogenous single-base insertion (c.3944-3945 ins C, p.T1315fsX1331) in exon 10. The patient exhibited severe early-onset phenotype and suffered from hemophagocytic lymphohistiocytosis. Patient 5 had two heterogenous nonsense mutations; c.7982C>G, p.S2661X in exon 30 and c.8281A>T, p.R2761X in exon 31. The patient suffered from infections in childhood and had visual disturbance and albinism of the skin and hair. The CHS1 mutations described here have not been reported previously.

Elevated oxidative membrane damage associated with genetic modifiers of Lyst-mutant phenotypes

LYST is a large cytosolic protein that influences the biogenesis of lysosome-related organelles, and mutation of the encoding gene, LYST, can cause Chediak-Higashi syndrome. Recently, Lyst-mutant mice were recognized to also exhibit an iris disease resembling exfoliation syndrome, a common cause of glaucoma in humans. Here, Lyst-mutant iris phenotypes were used in a search for genes that influence Lyst pathways. In a candidate gene–driven approach, albino Lyst-mutant mice homozygous for a mutation in Tyr, whose product is key to melanin synthesis within melanosomes, exhibited complete rescue of Lyst-mutant iris phenotypes. In a genetic background–driven approach using a DBA/2J strain of congenic mice, an interval containing Tyrp1 enhanced Lyst-dependent iris phenotypes. Thus, both experimental approaches implicated the melanosome, an organelle that is a potential source of oxidative stress, as contributing to the disease phenotype. Confirming an association with oxidative damage, Lyst mutation resulted in genetic context–sensitive changes in iris lipid hydroperoxide levels, being lowest in albino and highest in DBA/2J mice. Surprisingly, the DBA/2J genetic background also exposed a late-onset neurodegenerative phenotype involving cerebellar Purkinje-cell degeneration. These results identify an association between oxidative damage to lipid membranes and the severity of Lyst-mutant phenotypes, revealing a new mechanism that contributes to pathophysiology involving LYST.

LYST is a poorly understood protein involved in hereditary disease. Mutations in the encoding gene cause Chediak-Higashi syndrome, a rare lethal disease affecting multiple tissues of the body. Mutations in Lyst also recapitulate features of exfoliation syndrome, a common disease affecting the anterior chamber of the eye. Unfortunately, the Lyst gene is quite large, rendering it difficult to study by many molecular and cellular approaches. Here, we use a genetic approach in mice to identify additional genetic pathways which might modify, or prevent, the ill consequences associated with Lyst mutation. Our experiments demonstrate that Lyst mutation results in elevated levels of oxidative damage to lipid membranes. These results identify a previously unrecognized consequence of Lyst mutation and a modifiable pathway of potential clinical relevance in humans. Ultimately, knowledge of these events will contribute to the design of new therapeutic strategies allowing a similar alleviation of disease in humans.

Vesicle traffic to the immunological synapse: a multifunctional process targeted by lymphotropic viruses

The site of contact between T lymphocytes and antigen-presenting cells becomes, upon antigen recognition, an organized junction named the immunological synapse. Various T cell organelles polarize, together with microtubules, toward the antigen-presenting cell. Among them, intracellular vesicular compartments, such as the Golgi apparatus, the recycling endosomal compartment, or cytotoxic granules help to build the immunological synapse and ensure effector functions, such as polarized secretion of cytokines by helper T cells, or exocytosis of lytic granules by cytotoxic T cells. Lymphotropic retroviruses, such as the human immunodeficiency virus type 1, the human T cell leukemia virus type 1, or the Herpesvirus saimiri, can subvert some of the vesicle traffic mechanisms impeding the generation and function of the immunological synapses. This review focuses on the polarization of vesicle traffic, its regulation, and its role in maintaining the structure and function of the immunological synapse. We discuss how some lymphotropic viruses target the vesicle traffic in T lymphocytes, inhibiting the formation of immunological synapses and modulating the response of infected T cells.

Potential large animal models for gene therapy of human genetic diseases of immune and blood cell systems

Genetic mutations involving the cellular components of the hematopoietic system--red blood cells, white blood cells, and platelets--manifest clinically as anemia, infection, and bleeding. Although gene targeting has recapitulated many of these diseases in mice, these murine homologues are limited as translational models by their small size and brief life span as well as the fact that mutations induced by gene targeting do not always faithfully reflect the clinical manifestations of such mutations in humans. Many of these limitations can be overcome by identifying large animals with genetic diseases of the hematopoietic system corresponding to their human disease counterparts. In this article, we describe human diseases of the cellular components of the hematopoietic system that have counterparts in large animal species, in most cases carrying mutations in the same gene (CD18 in leukocyte adhesion deficiency) or genes in interacting proteins (DNA cross-link repair 1C protein and protein kinase, DNA-activated catalytic polypeptide in radiation-sensitive severe combined immunodeficiency). Furthermore, we describe the potential of these animal models to serve as disease-specific preclinical models for testing the efficacy and safety of clinical interventions such as hematopoietic stem cell transplantation or gene therapy before their use in humans with the corresponding disease.

Lyst mutation in mice recapitulates iris defects of human exfoliation syndrome

PURPOSE

Human eyes with exfoliation syndrome (XFS) exhibit a distinctive pattern of iris transillumination defects that are recapitulated in Lyst mutant mice carrying the beige allele. The purpose of this study was to determine the anatomic basis for Lyst-mediated transillumination defects, test whether Lyst mutant mice develop other features of XFS, and describe the molecular basis of the beige mutation.

METHODS

Lyst mutant mice and strain-matched controls were compared by clinical, histologic, immunohistochemical, and molecular genetic analyses.

RESULTS

Slit-lamp examination showed that Lyst mutant mice uniformly exhibit XFS-like transillumination defects. Histologic analysis showed that these defects correlate with a sawtooth morphology of the iris pigment epithelium. Lyst mutant mice also produce an exfoliative-like material and exhibit pronounced pigment dispersion. Despite these insults, Lyst mutation does not cause increased intraocular pressure or optic nerve damage in the C57BL/6J genetic background. Sequence analysis identified that the beige mutation is predicted to delete a single isoleucine from the WD40 domain of the LYST protein, suggesting that this mutation is likely to disrupt a protein-protein interaction.

CONCLUSIONS

Lyst mutant eyes exhibit multiple features of XFS. Recent human genetic association studies have identified changes occurring in the LOXL1 gene as an important risk factor for XFS but also indicated that other factors contributing to risk likely exist. These results demonstrated that mutation of the Lyst gene can produce ocular features of human XFS and suggested that LYST or LYST-interacting genes may contribute to XFS.

The integral membrane of lysosomes: its proteins and their roles in disease

The protein composition of the integral lysosomal membrane and the membrane-associated compartment have been defined in part by proteomics approaches. While the role of its constituent hydrolases in a large array of human disorders has been well-documented, the manner in which membrane proteins are integrated into the organelle, the multiprotein complexes that form at the organelle's cytosolic surface and their roles in the biogenesis and functional control of the organelle are now emerging. Defining cytosolic targeting complexes that affect the function of the lysosomal/endosomal compartment may help to identify the lysosome's role in a variety of human pathologies.

Control of parasitophorous vacuole expansion by LYST/Beige restricts the intracellular growth of Leishmania amazonensis

The intracellular protozoan Leishmania replicates in parasitophorous vacuoles (PV) that share many features with late endosomes/lysosomes. L. amazonensis PVs expand markedly during infections, but the impact of PV size on parasite intracellular survival is still unknown. Here we show that host cells infected with L. amazonensis upregulate transcription of LYST/Beige, which was previously shown to regulate lysosome size. Mutations in LYST/Beige caused further PV expansion and enhanced L. amazonensis replication. In contrast, LYST/Beige overexpression led to small PVs that did not sustain parasite growth. Treatment of LYST/Beige over-expressing cells with vacuolin-1 reversed this phenotype, expanding PVs and promoting parasite growth. The opposite was seen with E-64d, which reduced PV size in LYST-Beige mutant cells and inhibited L. amazonensis replication. Enlarged PVs appear to protect parasites from oxidative damage, since inhibition of nitric oxide synthase had no effect on L. amazonensis viability within large PVs, but enhanced their growth within LYST/Beige-induced small PVs. Thus, the upregulation of LYST/Beige in infected cells functions as a host innate response to limit parasite growth, by reducing PV volume and inhibiting intracellular survival.

The protozoan parasite Leishmania causes serious infections in humans throughout the world. After being inoculated into the skin through the bite of infected sandflies, the parasites enter host cells and replicate. The lysosome-like intracellular vacuoles where Leishmania amazonensis replicates expand dramatically as the infection progresses. Here we studied the impact of vacuole expansion on the ability of the parasites to survive and replicate inside host cells. We found that the host cell responds to infection with Leishmania amazonensis by upregulating expression of LYST/Beige, a gene that regulates the size of lysosomes and of parasite-containing vacuoles. The parasites replicated more efficiently in the large vacuoles formed in cells that have a mutation in LYST/Beige, whereas the same cells overexpressing functional LYST/Beige generated small vacuoles that were not able to sustain parasite growth. Drug treatments that reduced or enhanced the size of parasite-containing vacuoles had a corresponding effect on intracellular replication, demonstrating that large vacuoles provide a growth advantage to Leishmania amazonensis. Our results indicate that host cells respond to Leishmania infections by producing a protein capable of reducing vacuole size, as a strategy to inhibit parasite growth.

Purification and proteomic analysis of lysosomal integral membrane proteins

Lysosomes are essential for normal function of cells. This is best illustrated by the occurrence of greater than 40 lysosomal storage diseases. While the enzymes of the luminal compartment have been widely studied usually in the context of these diseases, the composition of the enveloping membrane has received scant attention. Advances in mass spectrometry and proteomics have laid the necessary groundwork to facilitate investigation of membranes such as those of lysosomes, mitochondria, and other organelles to find novel proteins and novel functions. Pure lysosomes are a prerequisite, and we have successfully identified an abundance of membrane proteins from lysosomes of rat liver. Here, we describe two comparable and easy methods to isolate lysosomes from mouse or rat liver in sufficient quantities for proteomics studies. Also included is a comparison of the soluble, luminal proteins obtained from each of the two preparations separated by 2D immobilized pH gradient (IPG) sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE).

Secretory mechanisms in cell-mediated cytotoxicity

Cytotoxic T lymphocytes (CTLs) play a critical role in the immune system; they are able to recognize and destroy virally infected and tumorigenic cells. Specific recognition of MHC class I-peptide complexes by the T cell receptor (TcR) results in precise delivery of lytic granules to the target cell, sparing neighboring cells and the CTL itself. Over the past 10 years various studies have eludicated the mechanisms that lead to the rapid polarization of the secretory apparatus in CTLs. These studies highlight similarities and differences between polarity and secretory mechanisms seen in other cell types and developmental systems. This review focuses on recent advances in our understanding of the molecular basis of polarized secretion from CTLs and the novel mechanism used by these cells to deliver their lethal hit.

Effect of a thiol proteinase inhibitor, E-64-d, on susceptibility to infection with Staphylococcus aureus in Chediak–Higashi syndrome (beige) mice

We previously reported that abnormally down-regulated protein kinase C (PKC) activity is responsible for the impaired cellular function of natural killer cells and polymorphonuclear cells (PMNs), and the giant granule formation in fibroblasts in the beige mouse, an animal model of Chediak-Higashi syndrome. Here, we examine the effect of oral or intraperitoneal administration of E-64-d, which protects PKC from calpain-mediated proteolysis, on the impaired cellular function in PMNs from beige mice. We found that oral administration of E-64-d (12.5 mg/kg body weight per day) for three consecutive days, significantly improved the abnormally increased concanavalin A (Con A) cap formation and the decreased lysosomal enzyme activity in beige PMNs. In addition, E-64-d significantly improved the delayed bactericidal activity against Staphylococcus aureus. In contrast, E-64-d at the same dose did not affect these cellular functions in PMNs from C57BL/6J mice. We confirmed that the abnormal down-regulation of PKC after Con A stimulation was eliminated in PMNs from E-64-d-treated beige PMNs. We then examined whether the administration of E-64-d to beige mice improved the susceptibility to experimental infection with S. aureus (2x10(8)/mouse). Both intraperitoneal and oral administration of E-64-d to beige mice resulted in a significant increase in survival, whereas E-64-d at the same dose did not alter the survival rate in normal mice. These results suggest that the administration of E-64-d may be effective against severe bacterial infection in Chediak-Higashi syndrome.

The BEACH Protein LvsB Is Localized on Lysosomes and Postlysosomes and Limits Their Fusion with Early Endosomes

The Chediak-Higashi syndrome (CHS) is a genetic disorder caused by the loss of the BEACH protein Lyst. Impaired lysosomal function in CHS patients results in many physiological problems, including immunodeficiency, albinism and neurological problems. Dictyostelium LvsB is the ortholog of mammalian Lyst and is also important for lysosomal function. A knock-in approach was used to tag LvsB with green fluorescent protein (GFP) and express it from its single chromosomal locus. GFP-LvsB was observed on late lysosomes and postlysosomes. Loss of LvsB resulted in enlarged postlysosomes, in the abnormal localization of proton pumps on postlysosomes and their abnormal acidification. The abnormal postlysosomes in LvsB-null cells were produced by the inappropriate fusion of early endosomal compartments with postlysosomal compartments. The intermixing of compartments resulted in a delayed transit of fluid-phase marker through the endolysosomal system. These results support the model that LvsB and Lyst proteins act as negative regulators of fusion by limiting the heterotypic fusion of early endosomes with postlysosomal compartments.

The WD repeat protein FAN regulates lysosome size independent from abnormal downregulation/membrane recruitment of protein kinase C

FAN (factor associated with neutral sphingomyelinase [N-SMase] activation) exhibits striking structural homologies to Lyst (lysosomal trafficking regulator), a BEACH protein whose inactivation causes formation of giant lysosomes/Chediak-Higashi syndrome. Here, we show that cells lacking FAN show a statistically significant increase in lysosome size (although less pronounced as Lyst), pointing to previously unrecognized functions of FAN in regulation of the lysosomal compartment. Since FAN regulates activation of N-SMase in complex with receptor for activated C-kinase (RACK)1, a scaffolding protein that recruits and stabilizes activated protein kinase C (PKC) isotypes at cellular membranes, and since an abnormal (calpain-mediated) downregulation/membrane recruitment of PKC has been linked to the defects observed in Lyst-deficient cells, we assessed whether PKC is also of relevance in FAN signaling. Our results demonstrate that activation of PKC is not required for regulation of N-SMase by FAN/RACK1. Conversely, activation of PKC and recruitment/stabilization by RACK1 occurs uniformly in the presence or absence of FAN (and equally, Lyst). Furthermore, regulation of lysosome size by FAN is not coupled to an abnormal downregulation/membrane recruitment of PKC by calpain. Identical results were obtained for Lyst, questioning the previously reported relevance of PKC for formation of giant lysosomes and in Chediak-Higashi syndrome. In summary, FAN mediates activation of N-SMase as well as regulation of lysosome size by signaling pathways that operate independent from activation/membrane recruitment of PKC.

When intracellular logistics fails--genetic defects in membrane trafficking

The number of human genetic disorders shown to be due to defects in membrane trafficking has greatly increased during the past five years. Defects have been identified in components involved in sorting of cargo into transport carriers, vesicle budding and scission, movement of vesicles along cytoskeletal tracks, as well as in vesicle tethering, docking and fusion at the target membrane. The nervous system is extremely sensitive to such disturbances of the membrane trafficking machinery, and the majority of these disorders display neurological defects--particularly diseases affecting the motility of transport carriers along cytoskeletal tracks. In several disorders, defects in a component that represents a fundamental part of the trafficking machinery fail to cause global transport defects but result in symptoms limited to specific cell types and transport events; this apparently reflects the redundancy of the transport apparatus. In groups of closely related diseases such as Hermansky-Pudlak and Griscelli syndromes, identification of the underlying gene defects has revealed groups of genes in which mutations lead to similar phenotypic consequences. New functionally linked trafficking components and regulatory mechanisms have thus been discovered. Studies of the gene defects in trafficking disorders therefore not only open avenues for new therapeutic approaches but also significantly contribute to our knowledge of the fundamental mechanisms of intracellular membrane transport.

Lysosomal Membranes from Beige Mice Contain Higher Than Normal Levels of Endoplasmic Reticulum Proteins

Chediak-Higashi syndrome is characterized by dysfunctional giant organelles of common origin, that is, lysosomes, melanosomes, and platelet dense bodies. Its defective gene LYST encodes a large molecular weight protein whose function is unknown. The Beige mouse also defective in Lyst is a good model of the human disease. Purified lysosomes from Beige and normal black mouse livers were used to carry out a proteomics study. Two-dimensional gel electrophoretic separation of soluble lysosomal proteins of Beige and normal mice revealed no major differences. The cleavable isotope-coded affinity tag (cICAT) technique was used to compare the composition of Beige and normal lysosomal membrane proteins. While the levels of common proteins, that is, Lamp1, Lamp2, and Niemann-Pick type C1, were decreased in Beige mice, there was an increase in the levels of endoplasmic reticulum (ER) resident proteins, for example, cytochrome P450, NADPH-cytochrome P450 oxidoreductase, and flavin-containing monooxygenase. Confocal microscopy confirmed that another ER protein, calnexin, colocalizes with Lamp1 on membranes of giant lysosomes from fibroblasts of Chediak-Higashi syndrome patient. Our results suggest that LYST may play a role in either preventing inappropriate incorporation of proteins into the lysosomal membrane or in membrane recycling/maturation.

A missense mutation in the WD40 domain of murine Lyst is linked to severe progressive Purkinje cell degeneration

Disturbance of intracellular trafficking plays a major role in several neurodegenerative disorders including Alzheimer or Parkinson's disease. The Chediak-Higashi syndrome (CHS), a life-threatening autosomal recessive disease with frequent mutations in the LYST gene, and its animal model, the beige mouse, are both characterized by lysosomal defects with accumulation of giant lysosomes. Clinically they manifest as hypopigmentation, abnormal bleeding and increased susceptibility to infection with various degrees of involvement of the nervous system. In the course of a recessive N-ethyl-N-nitrosurea (ENU) mutagenesis screen, we identified the first murine missense mutation in the lysosomal trafficking regulator gene (Lyst(Ing3618)) located at a highly conserved position in the WD40 protein domain. Nearly all described human Lyst alleles lead to protein truncation and fatal childhood CHS. Only four different missense mutations have been reported in patients with adolescent or adult forms of CHS involving the nervous system. Interestingly, the Lyst(Ing3618) model presents with a predominant neurodegenerative phenotype with progressive degeneration and loss of Purkinje cells and lacks severe impairment of the immune system. Therefore, the Lyst(Ing3618 )allele could represent a new model for adult CHS with neurological impairment. It could also provide an important tool to elucidate the role of neuronal lysosomal trafficking in the pathophysiology of neurodegeneration.

Grey, a novel mutation in the murine Lyst gene, causes the beige phenotype by skipping of exon 25

The murine beige mutant phenotype and the human Chediak-Higashi syndrome are caused by mutations in the murine Lyst (lysosomal trafficking regulator) gene and the human CHS gene, respectively. In this report we have analyzed a novel murine mutant Lyst allele, called Lyst(bg-grey), that had been found in an ENU mutation screen and named grey because of the grey coat color of affected mice. The phenotype caused by the Lyst(bg-grey) mutation was inherited in a recessive fashion. Melanosomes of melanocytes associated with hair follicles and the choroid layer of the eye, as well as melanosomes in the neural tube-derived pigment epithelium of the retina, were larger and irregularly shaped in homozygous mutants compared with those of wild-type controls. Secretory vesicles in dermal mast cells of the mutant skin were enlarged as well. Test crosses with beige homozygous mutant mice (Lyst(bg)) showed that double heterozygotes (Lyst(bg)/Lyst(bg-grey)) were phenotypically indistinguishable from either homozygous parent, demonstrating that the ENU mutation was an allele of the murine Lyst gene. RT-PCR analyses revealed the skipping of exon 25 in Lyst(bg-grey) mutants, which is predicted to cause a missense D2399E mutation and the loss of the following 77 amino acids encoded by exon 25 but leave the C-terminal end of the protein intact. Analysis of the genomic Lyst locus around exon 25 showed that the splice donor at the end of exon 25 showed a T-to-C transition point mutation. Western blot analysis suggests that the Lyst(bg-grey) mutation causes instability of the LYST protein. Because the phenotype of Lyst(bg) and Lyst(bg-grey) mutants is indistinguishable, at least with respect to melanosomes and secretory granules in mast cells, the Lyst(bg-grey) mutation defines a critical region for the stability of the murine LYST protein.

Das Chediak-Higashi-Syndrom

Chediak-Higashi syndrome (CHS) is a rare autosomal recessive lysosomal disorder characterized by frequent infections, oculocutaneous albinism, high bleeding tendency, and various neurological symptoms. Onset in early childhood mostly leads to lymphohistiocytic infiltration into multiple organs, which is usually lethal without bone marrow transplantation. The adult form of CHS has a milder course, no lymphohistiocytic infiltration, and is characterized by neurological manifestations such as polyneuropathy, parkinsonism, dementia, and ataxia. In young adults, a combination of these defects with oculocutaneous albinism or recurrent infections should bring CHS into consideration. Diagnosis is established by the presence of characteristic eosinophilic peroxidase-positive giant granules in leukocytes. This article summarizes current knowledge about the pathogenesis, clinical course, and therapy of CHS and reports on experience with two adult CHS patients.