Characterization of common SMPD1 mutations causing types A and B Niemann-Pick disease and generation of mutation-specific mouse models

Polymer-based drug delivery systems under investigation for enzyme replacement and other therapies of lysosomal storage disorders

Lysosomes play a central role in cellular homeostasis and alterations in this compartment associate with many diseases. The most studied example is that of lysosomal storage disorders (LSDs), a group of 60 + maladies due to genetic mutations affecting lysosomal components, mostly enzymes. This leads to aberrant intracellular storage of macromolecules, altering normal cell function and causing multiorgan syndromes, often fatal within the first years of life. Several treatment modalities are available for a dozen LSDs, mostly consisting of enzyme replacement therapy (ERT) strategies. Yet, poor biodistribution to main targets such as the central nervous system, musculoskeletal tissue, and others, as well as generation of blocking antibodies and adverse effects hinder effective LSD treatment. Drug delivery systems are being studied to surmount these obstacles, including polymeric constructs and nanoparticles that constitute the focus of this article. We provide an overview of the formulations being tested, the diseases they aim to treat, and the results observed from respective in vitro and in vivo studies. We also discuss the advantages and disadvantages of these strategies, the remaining gaps of knowledge regarding their performance, and important items to consider for their clinical translation. Overall, polymeric nanoconstructs hold considerable promise to advance treatment for LSDs.

The Niemann-Pick type diseases – A synopsis of inborn errors in sphingolipid and cholesterol metabolism

Disturbances of lipid homeostasis in cells provoke human diseases. The elucidation of the underlying mechanisms and the development of efficient therapies represent formidable challenges for biomedical research. Exemplary cases are two rare, autosomal recessive, and ultimately fatal lysosomal diseases historically named "Niemann-Pick" honoring the physicians, whose pioneering observations led to their discovery. Acid sphingomyelinase deficiency (ASMD) and Niemann-Pick type C disease (NPCD) are caused by specific variants of the sphingomyelin phosphodiesterase 1 (SMPD1) and NPC intracellular cholesterol transporter 1 (NPC1) or NPC intracellular cholesterol transporter 2 (NPC2) genes that perturb homeostasis of two key membrane components, sphingomyelin and cholesterol, respectively. Patients with severe forms of these diseases present visceral and neurologic symptoms and succumb to premature death. This synopsis traces the tortuous discovery of the Niemann-Pick diseases, highlights important advances with respect to genetic culprits and cellular mechanisms, and exposes efforts to improve diagnosis and to explore new therapeutic approaches.

Models to study basic and applied aspects of lysosomal storage disorders

The lack of available treatments and fatal outcome in most lysosomal storage disorders (LSDs) have spurred research on pathological mechanisms and novel therapies in recent years. In this effort, experimental methodology in cellular and animal models have been developed, with aims to address major challenges in many LSDs such as patient-to-patient variability and brain condition. These techniques and models have advanced knowledge not only of LSDs but also for other lysosomal disorders and have provided fundamental insights into the biological roles of lysosomes. They can also serve to assess the efficacy of classical therapies and modern drug delivery systems. Here, we summarize the techniques and models used in LSD research, which include both established and recently developed in vitro methods, with general utility or specifically addressing lysosomal features. We also review animal models of LSDs together with cutting-edge technology that may reduce the need for animals in the study of these devastating diseases.

Neonatal cholestasis and Niemann-pick type C disease: A literature review

BACKGROUND

Neonatal cholestasis (NC) is one of the most serious diseases in newborns and infants and results from metabolic disorders, such as Niemann-Pick type C (NPC), among other causes.

OBJECTIVE

We evaluated the incidence of NPC in our NC plus lysosomal storage disease (LSD) suspicious neonates and infants series.

METHODS

The study included children (≤3 years old) with a history of NC together with a suspicion of LSD, referred from Spanish Hospitals during the period 2011-2020. Screening for NPC was done by plasma biomarker assay (chitotriosidase activity and 7-ketocholesterol), and Sanger sequencing for NPC1 and NPC2 genes.

RESULTS

We screened NPC disease in 17 patients with NC plus organomegaly and that were LSD suspicious, finding 5 NPC patients (29.4%) and 2 carriers.

CONCLUSIONS

Our results emphasize the need to study NPC when NC and visceral enlargement arise in a newborn or infant.

An Early-Onset Neuronopathic Form of Acid Sphingomyelinase Deficiency: A SMPD1 p.C133Y Mutation in the Saposin Domain of Acid Sphingomyelinase

Acid sphingomyelinase (ASM) is a lysosomal hydrolase that degrades sphingomyelin into ceramide and phosphocholine. Recent crystallographic studies revealed the functional role of the N-terminal ASM saposin domain. ASM deficiency due to mutations in the ASM-encoding sphingomyelin phosphodiesterase 1 (SMPD1) gene causes an autosomal recessive sphingolipid-storage disorder, known as Niemann-Pick disease Type A (NPA) or Type B (NPB). NPA is an early-onset neuronopathic disorder, while NPB is a late-onset non-neuronopathic disorder. A homozygous one-base substitution (c.398G>A) of the SMPD1 gene was identified in an infant with NPA, diagnosed with complete loss of ASM activity in the patient's fibroblasts. This mutation is predicted to substitute tyrosine for cysteine at amino acid residue 133, abbreviated as p.C133Y. The patient showed developmental delay, hepatosplenomegaly and rapid neurological deterioration leading to death at the age of 3 years. To characterize p.C133Y, which may disrupt one of the three disulfide bonds of the N-terminal ASM saposin domain, we performed immunoblotting analysis to explore the expression of a mutant ASM protein in the patient's fibroblasts, showing that the protein was detected as a 70-kDa protein, similar to the wild-type ASM protein. Furthermore, transient expression of p.C133Y ASM protein in COS-7 cells indicated complete loss of ASM enzyme activity, despite that the p.C133Y ASM protein was properly localized to the lysosomes. These results suggest that the proper three-dimensional structure of saposin domain may be essential for ASM catalytic activity. Thus, p.C133Y is associated with complete loss of ASM activity even with stable protein expression and proper subcellular localization.

Characterization of Niemann-Pick diseases genes mutation spectrum in Iran and identification of a novel mutation in SMPD1 gene

Background: Niemann-Pick diseases are rare inherited lipid storage disorders caused by mutations in the SMPD1, NPC1, and NPC2 genes. The aim of this study was to assess the mutation spectrum of a cohort of Iranian Niemann-Pick patients.

Methods: A consanguineous couple with a child suspected of having Niemann-Pick disease type A (died at age 2) was screened for gene mutations in the SMPD1 gene. Sanger sequencing was performed for all exons and exon-intron boundary regions. A literature review on SMPD1, NPC1, and NPC2 genes mutations in Iran was conducted using published original papers on this subject.

Results: A novel frameshift c.762delG (p.Leu256fs*) at a heterozygous state was identified in the parents. According to the review study, identified mutations in 39 Iranian patients were concentrated in exon 2 of the SMPD1 gene and exons 8 and 9 of the NPC1 gene.

Conclusion: Niemann-Pick diseases genes mutation analysis (SMPD1, NPC1, and NPC2) in Iran shows the genetic heterogeneity of these diseases in this country. More studies with larger sample sizes should be conducted to further examine genetic changes associated with Niemann-Pick diseases in Iran.

Insights into the Molecular Mechanisms of Cholesterol Binding to the NPC1 and NPC2 Proteins

In recent years, a growing number of studies have implicated the coordinated action of NPC1 and NPC2 in intralysosomal transport and efflux of cholesterol. Our current understanding of this process developed with just over two decades of research. Since the cloning of the genes encoding the NPC1 and NPC2 proteins, studies of the biochemical defects observed when either gene is mutated along with computational and structural studies have unraveled key steps in the underlying mechanism. Here, we summarize the major contributions to our understanding of the proposed cholesterol transport controlled by NPC1 and NPC2, and briefly discuss recent findings of cholesterol binding and transport proteins beyond NPC1 and NPC2. We conclude with key questions and major challenges for future research on cholesterol transport by the NPC1 and NPC2 proteins.

Alterations in Lipid Profile of the Aging Kidney Identified by MALDI Imaging Mass Spectrometry

During aging, the kidney undergoes functional and physiological changes that are closely affiliated with chronic kidney disease (CKD). There is increasing evidence supporting the role of lipid or lipid-derived mediators in the pathogenesis of CKD and other aging-related diseases. To understand the role of lipids in various metabolic processes during kidney aging, we conducted matrix-assisted laser desorption/ionization-imaging mass spectrometry (MALDI-IMS) analysis in kidneys harvested from young (2 months old, n = 3) and old mice (24 months old, n = 3). MALDI-IMS analysis showed an increase in ceramide level and a decrease in sphingomyelin (SM) and phosphatidylcholine (PC) levels in kidneys of old mice. The increased expression of cPLA2 and SMPD1 protein in aged kidney was confirmed by immunohistochemistry and Western blot analysis. Our MALDI-IMS data showed the altered distribution of lipids in aged kidney as indicative of aging-related functional changes of the kidney. Combined analysis of MALDI-IMS and IHC confirmed lipidomic changes and expression levels of responsible enzymes as well as morphological changes.

Solving the secretory acid sphingomyelinase puzzle: Insights from lysosome-mediated parasite invasion and plasma membrane repair

Acid sphingomyelinase (ASM) is a lysosomal enzyme that cleaves the phosphorylcholine head group of sphingomyelin, generating ceramide. Recessive mutations in SMPD1, the gene encoding ASM, cause Niemann-Pick Disease Types A and B. These disorders are attributed not only to lipid accumulation inside lysosomes but also to changes on the outer leaflet of the plasma membrane, highlighting an extracellular role for ASM. Secretion of ASM occurs under physiological conditions, and earlier studies proposed two forms of the enzyme, one resident in lysosomes and another form that would be diverted to the secretory pathway. Such differential intracellular trafficking has been difficult to explain because there is only one SMPD1 transcript that generates an active enzyme, found primarily inside lysosomes. Unexpectedly, studies of cell invasion by the protozoan parasite Trypanosoma cruzi revealed that conventional lysosomes can fuse with the plasma membrane in response to elevations in intracellular Ca²⁺ , releasing their contents extracellularly. ASM exocytosed from lysosomes remodels the outer leaflet of the plasma membrane, promoting parasite invasion and wound repair. Here, we discuss the possibility that ASM release during lysosomal exocytosis, in response to various forms of stress, may represent a major source of the secretory form of this enzyme.

The role of acid sphingomyelinase and modulation of sphingolipid metabolism in bacterial infection

Acid sphingomyelinase (ASM) is a key enzyme in sphingolipid metabolism that converts sphingomyelin to ceramide, thereby modulating membrane structures and signal transduction. Bacterial pathogens can manipulate ASM activity and function, and use host sphingolipids during multiple steps of their infection process. An increase in ceramides upon infection results in the formation of ceramide-enriched membrane platforms that serve to cluster receptor molecules and organize intracellular signaling molecules, thus facilitating bacterial uptake. In this review, we focus on how extracellular bacterial pathogens target ASM and modulate membrane properties and signaling pathways to gain entry into eukaryotic cells or induce cell death. We describe how intracellular pathogens interfere with the intralysosomal functions of ASM to favor replication and survival. In addition, bacteria utilize their own sphingomyelinases as virulence factors to modulate sphingolipid metabolism. The potential of ASM as a target for treating bacterial infections is also discussed.

A Robust Liposomal Platform for Direct Colorimetric Detection of Sphingomyelinase Enzyme and Inhibitors

The enzyme sphingomyelinase (SMase) is an important biomarker for several diseases such as Niemann Pick's, atherosclerosis, multiple sclerosis, and HIV. We present a two-component colorimetric SMase activity assay that is more sensitive and much faster than currently available commercial assays. Herein, SMase-triggered release of cysteine from a sphingomyelin (SM)-based liposome formulation with 60 mol % cholesterol causes gold nanoparticle (AuNP) aggregation, enabling colorimetric detection of SMase activities as low as 0.02 mU/mL, corresponding to 1.4 pM concentration. While the lipid composition offers a stable, nonleaky liposome platform with minimal background signal, high specificity toward SMase avoids cross-reactivity of other similar phospholipases. Notably, use of an SM-based liposome formulation accurately mimics the natural in vivo substrate: the cell membrane. We studied the physical rearrangement process of the lipid membrane during SMase-mediated hydrolysis of SM to ceramide using small- and wide-angle X-ray scattering. A change in lipid phase from a liquid to gel state bilayer with increasing concentration of ceramide accounts for the observed increase in membrane permeability and consequent release of encapsulated cysteine. We further demonstrated the effectiveness of the sensor in colorimetric screening of small-molecule drug candidates, paving the way for the identification of novel SMase inhibitors in minutes. Taken together, the simplicity, speed, sensitivity, and naked-eye readout of this assay offer huge potential in point-of-care diagnostics and high-throughput drug screening.

Metabolic etiologies in West syndrome

West syndrome (WS) is an early life epileptic encephalopathy associated with infantile spasms, interictal electroencephalography (EEG) abnormalities including high amplitude, disorganized background with multifocal epileptic spikes (hypsarrhythmia), and often neurodevelopmental impairments. Approximately 64% of the patients have structural, metabolic, genetic, or infectious etiologies and, in the rest, the etiology is unknown. Here we review the contribution of etiologies due to various metabolic disorders in the pathology of WS. These may include metabolic errors in organic molecules involved in amino acid and glucose metabolism, fatty acid oxidation, metal metabolism, pyridoxine deficiency or dependency, or acidurias in organelles such as mitochondria and lysosomes. We discuss the biochemical, clinical, and EEG features of these disorders as well as the evidence of how they may be implicated in the pathogenesis and treatment of WS. The early recognition of these etiologies in some cases may permit early interventions that may improve the course of the disease.

High lumenal chloride in the lysosome is critical for lysosome function

Lysosomes are organelles responsible for the breakdown and recycling of cellular machinery. Dysfunctional lysosomes give rise to lysosomal storage disorders as well as common neurodegenerative diseases. Here, we use a DNA-based, fluorescent chloride reporter to measure lysosomal chloride in Caenorhabditis elegans as well as murine and human cell culture models of lysosomal diseases. We find that the lysosome is highly enriched in chloride, and that chloride reduction correlates directly with a loss in the degradative function of the lysosome. In nematodes and mammalian cell culture models of diverse lysosomal disorders, where previously only lysosomal pH dysregulation has been described, massive reduction of lumenal chloride is observed that is ~10³ fold greater than the accompanying pH change. Reducing chloride within the lysosome impacts Ca²⁺ release from the lysosome and impedes the activity of specific lysosomal enzymes indicating a broader role for chloride in lysosomal function.

DOI:http://dx.doi.org/10.7554/eLife.28862.001

In cells, worn out proteins and other unnecessary materials are sent to small compartments called lysosomes to be broken down and recycled. Lysosomes contain many different proteins including some that break down waste material into recyclable fragments and others that transport the fragments out of the lysosome. If any of these proteins do not work, waste products build up and cause disease. There are around 70 such lysosomal storage diseases, each arising from a different lysosomal protein not working correctly.

A recently developed “nanodevice” called Clensor can measure the levels of chloride ions inside cells. Clensor is constructed from DNA, and its fluorescence changes when it detects chloride ions. Although chloride ions have many biological roles, chloride ion levels had not been measured inside a living organism. Now, Chakraborty et al. – including some of the researchers who developed Clensor – have used this nanodevice to examine chloride ion levels in the lysosomes of the roundworm Caenorhabditis elegans. This revealed that the lysosomes contain high levels of chloride ions. Furthermore, reducing the amount of chloride in the lysosomes made them worse at breaking down waste.

Do lysosomes affected by lysosome storage diseases also contain low levels of chloride ions? To find out, Chakraborty et al. used Clensor to study C. elegans worms and mouse and human cells whose lysosomes accumulate waste products. In all these cases, the levels of chloride in the diseased lysosomes were much lower than normal. This had a number of effects on how the lysosomes worked, such as reducing the activity of key lysosomal proteins.

Chakraborty et al. also found that Clensor can be used to distinguish between different lysosomal storage diseases. This means that in the future, Clensor (or similar methods that directly measure chloride ion levels in lysosomes) may be useful not just for research purposes. They may also be valuable for diagnosing lysosomal storage diseases early in infancy that, if left undiagnosed, are fatal.

DOI:http://dx.doi.org/10.7554/eLife.28862.002

N-glycosylation of human sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A) is essential for stability, secretion and activity

Sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A) is a recently identified phosphodiesterase, which is a secreted N-linked glycoprotein. SMPDL3A is highly homologous to acid sphingomyelinase (aSMase), but unlike aSMase cannot cleave sphingomyelin. Rather, SMPDL3A hydrolyzes nucleotide tri- and diphosphates and their derivatives. While recent structural studies have shed light on these unexpected substrate preferences, many other aspects of SMPDL3A biology, which may give insight into its function in vivo, remain obscure. Here, we investigate the roles of N-glycosylation in the expression, secretion and activity of human SMPDL3A, using inhibitors of N-glycosylation and site-directed mutagenesis, with either THP-1 macrophages or CHO cells expressing human SMPDL3A. Tunicamycin (TM) treatment resulted in expression of non-glycosylated SMPDL3A that was not secreted, and was largely degraded by the proteasome. Proteasomal inhibition restored levels of SMPDL3A in TM-treated cells, although this non-glycosylated protein lacked phosphodiesterase activity. Enzymatic deglycosylation of purified recombinant SMPDL3A also resulted in significant loss of phosphodiesterase activity. Site-directed mutagenesis of individual N-glycosylation sites in SMPDL3A identified glycosylation of Asn69 and Asn222 as affecting maturation of its N-glycans and secretion. Glycosylation of Asn356 in SMPDL3A, an N-linked site conserved throughout the aSMase-like family, was critical for protection against proteasomal degradation and preservation of enzymatic activity. We provide the first experimental evidence for a predicted 22 residue N-terminal signal peptide in SMPDL3A, which is essential for facilitating glycosylation and is removed from the mature protein secreted from CHO cells. In conclusion, site-specific N-glycosylation is essential for the intracellular stability, secretion and activity of human SMPDL3A.

Types A and B Niemann-Pick disease

The eponym Niemann-Pick disease (NPD) refers to a group of patients who present with varying degrees of lipid storage and foam cell infiltration in tissues, as well as overlapping clinical features including hepatosplenomegaly, pulmonary insufficiency and/or central nervous system (CNS) involvement. Due to the pioneering work of Roscoe Brady and co-workers, we now know that there are two distinct metabolic abnormalities that account for NPD. The first is due to the deficient activity of the enzyme acid sphingomyelinase (ASM; "types A & B" NPD), and the second is due to defective function in cholesterol transport ("type C" NPD). Herein only types A and B NPD will be discussed. Type A NPD patients exhibit hepatosplenomegaly in infancy and profound CNS involvement. They rarely survive beyond 2-3years of age. Type B patients also have hepatosplenomegaly and pathologic alterations of their lungs, but there are usually no CNS signs. The age of onset and rate of disease progression varies greatly among type B patients, and they frequently live into adulthood. Intermediate patients also have been reported with mild to moderate neurological findings. All patients with types A and B NPD have mutations in the gene encoding ASM (SMPD1), and thus the disease is more accurately referred to as ASM deficiency (ASMD). Herein we will review the clinical, pathological, biochemical, and genetic findings in types A and B NPD, and emphasize the seminal contributions of Dr. Brady to this disease. We will also discuss the current status of therapy for this disorder.

Structural and functional analysis of the ASM p.Ala359Asp mutant that causes acid sphingomyelinase deficiency

Niemann-Pick disease (NPD) type A and B are recessive hereditary disorders caused by deficiency in acid sphingomyelinase (ASM). The p.Ala359Asp mutation has been described in several patients but its functional and structural effects in the protein are unknown. In order to characterize this mutation, we modeled the three-dimensional ASM structure using the recent available crystal of the mammalian ASM as a template. We found that the p.Ala359Asp mutation is localized in the hydrophobic core and far from the sphingomyelin binding site. However, energy function calculations using statistical potentials indicate that the mutation causes a decrease in ASM stability. Therefore, we investigated the functional effect of the p.Ala359Asp mutation in ASM expression, secretion, localization and activity in human fibroblasts. We found a 3.8% residual ASM activity compared to the wild-type enzyme, without changes in the other parameters evaluated. These results support the hypothesis that the p.Ala359Asp mutation causes structural alterations in the hydrophobic environment where ASM is located, decreasing its enzymatic activity. A similar effect was observed in other previously described NPDB mutations located outside the active site of the enzyme. This work shows the first full size ASM mutant model describe at date, providing a complete analysis of the structural and functional effects of the p.Ala359Asp mutation over the stability and activity of the enzyme.

Spectrum of SMPD1 mutations in Asian-Indian patients with acid sphingomyelinase (ASM)-deficient Niemann-Pick disease

Acid sphingomyelinase (ASM)-deficient Niemann-Pick disease is an autosomal recessive lysosomal storage disorder caused by biallelic mutations in the SMPD1 gene. To date, around 185 mutations have been reported in patients with ASM-deficient NPD world-wide, but the mutation spectrum of this disease in India has not yet been reported. The aim of this study was to ascertain the mutation profile in Indian patients with ASM-deficient NPD. We sequenced SMPD1 in 60 unrelated families affected with ASM-deficient NPD. A total of 45 distinct pathogenic sequence variants were found, of which 14 were known and 31 were novel. The variants included 30 missense, 4 nonsense, and 9 frameshift (7 single base deletions and 2 single base insertions) mutations, 1 indel, and 1 intronic duplication. The pathogenicity of the novel mutations was inferred with the help of the mutation prediction software MutationTaster, SIFT, Polyphen-2, PROVEAN, and HANSA. The effects of the identified sequence variants on the protein structure were studied using the structure modeled with the help of the SWISS-MODEL workspace program. The p. (Arg542*) (c.1624C>T) mutation was the most commonly identified mutation, found in 22% (26 out of 120) of the alleles tested, but haplotype analysis for this mutation did not identify a founder effect for the Indian population. To the best of our knowledge, this is the largest study on mutation analysis of patients with ASM-deficient Niemann-Pick disease reported in literature and also the first study on the SMPD1 gene mutation spectrum in India. © 2016 Wiley Periodicals, Inc.

The structure and catalytic mechanism of human sphingomyelin phosphodiesterase like 3a--an acid sphingomyelinase homologue with a novel nucleotide hydrolase activity

Human sphingomyelinase phosphodiesterase like 3a (SMPDL3a) is a secreted enzyme that shares a conserved catalytic domain with human acid sphingomyelinase (aSMase), the enzyme carrying mutations causative of Niemann-Pick disease. We have solved the structure of SMPDL3a revealing a calcineurin-like fold. A dimetal site, glycosylation pattern and a disulfide bond network are likely to be conserved also in human aSMase. We show that the binuclear site of SMPDL3a is occupied by two Zn(2+) ions and that excess Zn(2+) leads to inhibition of enzyme activity through binding to additional sites. As an extension of recent biochemical work we uncovered that SMPDL3a catalyses the hydrolysis of several modified nucleotides that include cytidine 5'-diphosphocholine, cytidine diphosphate ethanolamine and ADP-ribose, but not the aSMase substrate, sphingomyelin. We subsequently determined the structure of SMPDL3a in complex with the product 5'-cytidine monophosphate (CMP), a structure that is consistent with several distinct coordination modes of the substrate/product in the active site during the reaction cycle. Based on the structure of CMP complexes, we propose a phosphoryl transfer mechanism for SMPDL3a. Finally, a homology model of human aSMase was constructed to allow for the mapping of selected Niemann-Pick disease mutations on a three-dimensional framework to guide further characterization of their effects on aSMase function.

DATABASE

Structural data are available in the PDB database under the accession numbers 5EBB and 5EBE.

Genetics of Lipid-Storage Management in Caenorhabditis elegans Embryos

Lipids play a pivotal role in embryogenesis as structural components of cellular membranes, as a source of energy, and as signaling molecules. On the basis of a collection of temperature-sensitive embryonic lethal mutants, a systematic database search, and a subsequent microscopic analysis of >300 interference RNA (RNAi)-treated/mutant worms, we identified a couple of evolutionary conserved genes associated with lipid storage in Caenorhabditis elegans embryos. The genes include cpl-1 (cathepsin L-like cysteine protease), ccz-1 (guanine nucleotide exchange factor subunit), and asm-3 (acid sphingomyelinase), which is closely related to the human Niemann-Pick disease-causing gene SMPD1. The respective mutant embryos accumulate enlarged droplets of neutral lipids (cpl-1) and yolk-containing lipid droplets (ccz-1) or have larger genuine lipid droplets (asm-3). The asm-3 mutant embryos additionally showed an enhanced resistance against C band ultraviolet (UV-C) light. Herein we propose that cpl-1, ccz-1, and asm-3 are genes required for the processing of lipid-containing droplets in C. elegans embryos. Owing to the high levels of conservation, the identified genes are also useful in studies of embryonic lipid storage in other organisms.

SMPD1 Mutation Update: Database and Comprehensive Analysis of Published and Novel Variants

Niemann-Pick Types A and B (NPA/B) diseases are autosomal recessive lysosomal storage disorders caused by the deficient activity of acid sphingomyelinase (ASM) because of the mutations in the SMPD1 gene. Here, we provide a comprehensive updated review of already reported and newly identified SMPD1 variants. Among them, 185 have been found in NPA/B patients. Disease-causing variants are equally distributed along the SMPD1 gene; most of them are missense (65.4%) or frameshift (19%) mutations. The most frequently reported mutation worldwide is the p.R610del, clearly associated with an attenuated NP disease type B phenotype. The available information about the impact of 52 SMPD1 variants on ASM mRNA and/or enzymatic activity has been collected and whenever possible, phenotype/genotype correlations were established. In addition, we created a locus-specific database easily accessible at http://www.inpdr.org/genes that catalogs the 417 SMPD1 variants reported to date and provides data on their in silico predicted effects on ASM protein function or mRNA splicing. The information reviewed in this article, providing new insights into the genotype/phenotype correlation, is extremely valuable to facilitate diagnosis and genetic counseling of families affected by NPA/B.

Successful within-patient dose escalation of olipudase alfa in acid sphingomyelinase deficiency

BACKGROUND

Olipudase alfa, a recombinant human acid sphingomyelinase (rhASM), is an investigational enzyme replacement therapy (ERT) for patients with ASM deficiency [ASMD; Niemann-Pick Disease (NPD) A and B]. This open-label phase 1b study assessed the safety and tolerability of olipudase alfa using within-patient dose escalation to gradually debulk accumulated sphingomyelin and mitigate the rapid production of metabolites, which can be toxic. Secondary objectives were pharmacokinetics, pharmacodynamics, and exploratory efficacy.

METHODS

Five adults with nonneuronopathic ASMD (NPD B) received escalating doses (0.1 to 3.0 mg/kg) of olipudase alfa intravenously every 2 weeks for 26 weeks.

RESULTS

All patients successfully reached 3.0mg/kg without serious or severe adverse events. One patient repeated a dose (2.0 mg/kg) and another had a temporary dose reduction (1.0 to 0.6 mg/kg). Most adverse events (97%) were mild and all resolved without sequelae. The most common adverse events were headache, arthralgia, nausea and abdominal pain. Two patients experienced single acute phase reactions. No patient developed hypersensitivity or anti-olipudase alfa antibodies. The mean circulating half-life of olipudase alfa ranged from 20.9 to 23.4h across doses without accumulation. Ceramide, a sphingomyelin catabolite, rose transiently in plasma after each dose, but decreased over time. Reductions in sphingomyelin storage, spleen and liver volumes, and serum chitotriosidase activity, as well as improvements in infiltrative lung disease, lipid profiles, platelet counts, and quality of life assessments, were observed.

CONCLUSIONS

This study provides proof-of-concept for the safety and efficacy of within-patient dose escalation of olipudase alfa in patients with nonneuronopathic ASMD.

Epidemiological, clinical and biochemical characterization of the p.(Ala359Asp) SMPD1 variant causing Niemann-Pick disease type B

Niemann-Pick disease type B (NPDB) is a rare, inherited lysosomal storage disorder that occurs due to variants in the sphingomyelin phosphodiesterase 1 (SMPD1) gene and the resultant deficiency of acid sphingomyelinase (ASM) activity. While numerous variants causing NPDB have been described, only a small number have been studied in any detail. Herein, we describe the frequency of the p.(Ala359Asp) variant in the healthy Chilean population, and determine the haplotype background of homozygous patients to establish if this variant originated from a common founder. Genomic DNA samples from 1691 healthy individuals were analyzed for the p.(Ala359Asp) variant. The frequency of p.(Ala359Asp) was found to be 1/105.7, predicting a disease incidence of 1/44 960 in Chile, higher than the incidence estimated by the number of confirmed NPDB cases. We also describe the clinical characteristics of 13 patients homozygous for p.(Ala359Asp) and all of them had moderate to severe NPDB disease. In addition, a conserved haplotype and shared 280 Kb region around the SMPD1 gene was observed in the patients analyzed, indicating that the variant originated from a common ancestor. The haplotype frequency and mitochondrial DNA analysis suggest an Amerindian origin for the variant. To assess the effect of the p.(Ala359Asp) variant, we transfected cells with the ASM-p.(Ala359Asp) cDNA and the activity was only 4.2% compared with the wild-type cDNA, definitively demonstrating the causative effect of the variant on ASM function. Information on common variants such as p.(Ala359Asp) is essential to guide the successful implementation for future therapies and benefit to patients.

Types A and B Niemann-Pick disease

Two distinct metabolic abnormalities are encompassed under the eponym Niemann-Pick disease (NPD). The first is due to the deficient activity of the enzyme acid sphingomyelinase (ASM). Patients with ASM deficiency are classified as having types A and B Niemann-Pick disease (NPD). Type A NPD patients exhibit hepatosplenomegaly in infancy and profound central nervous system involvement. They rarely survive beyond two years of age. Type B patients also have hepatosplenomegaly and pathologic alterations of their lungs, but there are usually no central nervous system signs. The age of onset and rate of disease progression varies greatly among type B patients, and they frequently live into adulthood. Recently, patients with phenotypes intermediate between types A and B NPD also have been identified. These individuals represent the expected continuum caused by inheriting different mutations in the ASM gene (SMPD1). Patients in the second NPD category are designated as having types C and D NPD. These patients may have mild hepatosplenomegaly, but the central nervous system is profoundly affected. Impaired intracellular trafficking of cholesterol causes types C and D NPD, and two distinct gene defects have been found. In this chapter only types A and B NPD will be discussed.

Sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A) is a novel nucleotide phosphodiesterase regulated by cholesterol in human macrophages

Cholesterol-loaded foam cell macrophages are prominent in atherosclerotic lesions and play complex roles in both inflammatory signaling and lipid metabolism, which are underpinned by large scale reprogramming of gene expression. We performed a microarray study of primary human macrophages that showed that transcription of the sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A) gene is up-regulated after cholesterol loading. SMPDL3A protein expression in and secretion from primary macrophages are stimulated by cholesterol loading, liver X receptor ligands, and cyclic AMP, and N-glycosylated SMPDL3A protein is detectable in circulating blood. We demonstrate for the first time that SMPDL3A is a functional phosphodiesterase with an acidic pH optimum. We provide evidence that SMPDL3A is not an acid sphingomyelinase but unexpectedly is active against nucleotide diphosphate and triphosphate substrates at acidic and neutral pH. SMPDL3A is a major source of nucleotide phosphodiesterase activity secreted by liver X receptor-stimulated human macrophages. Extracellular nucleotides such as ATP may activate pro-inflammatory responses in immune cells. Increased expression and secretion of SMPDL3A by cholesterol-loaded macrophage foam cells in lesions may decrease local concentrations of pro-inflammatory nucleotides and potentially represent a novel anti-inflammatory axis linking lipid metabolism with purinergic signaling in atherosclerosis.

The Acid Sphingomyelinase Sequence Variant p.A487V Is Not Associated With Decreased Levels of Enzymatic Activity

Rare loss-of-function mutations in the sphingomyelin phosphodiesterase 1 (SMPD1) gene are known to dramatically decrease the catalytic activity of acid sphingomyelinase (ASM), resulting in an autosomal recessive lysosomal storage disorder known as Niemann-Pick disease (NPD) type A and B. In contrast to the general low frequency of those deleterious mutations, we found a relatively high frequency for the proposed type B NPD variant c.1460C>T (p.A487V) in our sample of 58 patients suffering from Major Depressive Disorder. We therefore investigated the biochemical consequences of this variant more closely. Our in vivo data derived from blood cell analyses indicated cellular ASM activity levels in the normal range. The secreted ASM activity levels in blood plasma were slightly lower, but still above those levels reported for type B NPD patients. In vitro expression studies of this ASM variant in different cell lines confirmed these results, showing cellular and secreted enzymatic activities equivalent to those of wild-type ASM and similar expression levels. Thus, we conclude that the ASM variant c.1460C>T (p.A487V) is not a rare missense mutation but an SMPD1 sequence variant that yields a protein with functional catalytic characteristics.

Cathepsin B overexpression due to acid sphingomyelinase ablation promotes liver fibrosis in Niemann-Pick disease

Niemann-Pick disease (NPD) is a lysosomal storage disease caused by the loss of acid sphingomyelinase (ASMase) that features neurodegeneration and liver disease. Because ASMase-knock-out mice models NPD and our previous findings revealed that ASMase activates cathepsins B/D (CtsB/D), our aim was to investigate the expression and processing of CtsB/D in hepatic stellate cells (HSCs) from ASMase-null mice and their role in liver fibrosis. Surprisingly, HSCs from ASMase-knock-out mice exhibit increased basal level and activity of CtsB as well as its in vitro processing in culture, paralleling the enhanced expression of fibrogenic markers α-smooth muscle actin (α-SMA), TGF-β, and pro-collagen-α1(I) (Col1A1). Moreover, pharmacological inhibition of CtsB blunted the expression of α-SMA and Col1A1 and proliferation of HSCs from ASMase-knock-out mice. Consistent with the enhanced activation of CtsB in HSCs from ASMase-null mice, the in vivo liver fibrosis induced by chronic treatment with CCl(4) increased in ASMase-null compared with wild-type mice, an effect that was reduced upon CtsB inhibition. In addition to liver, the enhanced proteolytic processing of CtsB was also observed in brain and lung of ASMase-knock-out mice, suggesting that the overexpression of CtsB may underlie the phenotype of NPD. Thus, these findings reveal a functional relationship between ASMase and CtsB and that the ablation of ASMase leads to the enhanced processing and activation of CtsB. Therefore, targeting CtsB may be of relevance in the treatment of liver fibrosis in patients with NPD.

Multivesicular bodies in neurons: distribution, protein content, and trafficking functions

Multivesicular bodies (MVBs) are intracellular endosomal organelles characterized by multiple internal vesicles that are enclosed within a single outer membrane. MVBs were initially regarded as purely prelysosomal structures along the degradative endosomal pathway of internalized proteins. MVBs are now known to be involved in numerous endocytic and trafficking functions, including protein sorting, recycling, transport, storage, and release. This review of neuronal MVBs summarizes their research history, morphology, distribution, accumulation of cargo and constitutive proteins, transport, and theories of functions of MVBs in neurons and glia. Due to their complex morphologies, neurons have expanded trafficking and signaling needs, beyond those of "geometrically simpler" cells, but it is not known whether neuronal MVBs perform additional transport and signaling functions. This review examines the concept of compartment-specific MVB functions in endosomal protein trafficking and signaling within synapses, axons, dendrites and cell bodies. We critically evaluate reports of the accumulation of neuronal MVBs based on evidence of stress-induced MVB formation. Furthermore, we discuss potential functions of neuronal and glial MVBs in development, in dystrophic neuritic syndromes, injury, disease, and aging. MVBs may play a role in Alzheimer's, Huntington's, and Niemann-Pick diseases, some types of frontotemporal dementia, prion and virus trafficking, as well as in adaptive responses of neurons to trauma and toxin or drug exposure. Functions of MVBs in neurons have been much neglected, and major gaps in knowledge currently exist. Developing truly MVB-specific markers would help to elucidate the roles of neuronal MVBs in intra- and intercellular signaling of normal and diseased neurons.

Brain pathology in Niemann Pick disease type A: insights from the acid sphingomyelinase knockout mice

Severe neurological involvement characterizes Niemann Pick disease (NPD) type A, an inherited disorder caused by loss of function mutations in the gene encoding acid sphingomyelinase (ASM). Mice lacking ASM, which mimic NPD type A, have provided important insights into the aberrant brain phenotypes induced by ASM deficiency. For example, lipid alterations, including the accumulation of sphingolipids, affect the membranes of different subcellular compartments of neurons and glial cells, leading to anomalies in signalling pathways, neuronal polarization, calcium homeostasis, synaptic plasticity, myelin production or immune response. These findings contribute to our understanding of the overall role of sphingolipids and their metabolic enzymes in brain physiology, and pave the way to design and test new therapeutic strategies for type A NPD and other neurodegenerative disorders. Some of these have already been tested in mice lacking ASM with promising results.

A novel mechanism of lysosomal acid sphingomyelinase maturation: requirement for carboxyl-terminal proteolytic processing

Acid sphingomyelinase (aSMase) catalyzes the hydrolysis of sphingomyelin (SM) to form the bioactive lipid ceramide (Cer). Notably, aSMase exists in two forms: a zinc (Zn(2+))-independent lysosomal aSMase (L-SMase) and a Zn(2+)-dependent secreted aSMase (S-SMase) that arise from alternative trafficking of a single protein precursor. Despite extensive investigation into the maturation and trafficking of aSMase, the exact identity of mature L-SMase has remained unclear. Here, we describe a novel mechanism of aSMase maturation involving C-terminal proteolytic processing within, or in close proximity to, endolysosomes. Using two different C-terminal-tagged constructs of aSMase (V5, DsRed), we demonstrate that aSMase is processed from a 75-kDa, Zn(2+)-activated proenzyme to a mature 65 kDa, Zn(2+)-independent L-SMase. L-SMase is recognized by a polyclonal Ab to aSMase, but not by anti-V5 or anti-DsRed antibodies, suggesting that the C-terminal tag is lost during maturation. Furthermore, indirect immunofluorescence staining demonstrated that mature L-SMase colocalized with the lysosomal marker LAMP1, whereas V5-aSMase localized to the Golgi secretory pathway. Moreover, V5-aSMase possessed Zn(2+)-dependent activity suggesting it may represent the common protein precursor of S-SMase and L-SMase. Importantly, the 65-kDa L-SMase, but not V5-aSMase, was sensitive to the lysosomotropic inhibitor desipramine, co-fractionated with lysosomes, and migrated at the same M(r) as partially purified human aSMase. Finally, three aSMase mutants containing C-terminal Niemann-Pick mutations (R600H, R600P, ΔR608) exhibited defective proteolytic maturation. Taken together, these results demonstrate that mature L-SMase arises from C-terminal proteolytic processing of pro-aSMase and suggest that impaired C-terminal proteolysis may lead to severe defects in L-SMase function.

Regulated secretion of acid sphingomyelinase: implications for selectivity of ceramide formation

The acid sphingomyelinase (aSMase) gene gives rise to two distinct enzymes, lysosomal sphingomyelinase (L-SMase) and secretory sphingomyelinase (S-SMase), via differential trafficking of a common protein precursor. However, the regulation of S-SMase and its role in cytokine-induced ceramide formation remain ill defined. To determine the role of S-SMase in cellular sphingolipid metabolism, MCF7 breast carcinoma cells stably transfected with V5-aSMase(WT) were treated with inflammatory cytokines. Interleukin-1β and tumor necrosis factor-α induced a time- and dose-dependent increase in S-SMase secretion and activity, coincident with selective elevations in cellular C(16)-ceramide. To establish a role for S-SMase, we utilized a mutant of aSMase (S508A) that is shown to retain L-SMase activity, but is defective in secretion. MCF7 expressing V5-aSMase(WT) exhibited increased S-SMase and L-SMase activity, as well as elevated cellular levels of specific long-chain and very long-chain ceramide species relative to vector control MCF7. Interestingly, elevated levels of only certain very long-chain ceramides were evident in V5-aSMase(S508A) MCF7. Secretion of the S508A mutant was also defective in response to IL-1β, as was the regulated generation of C(16)-ceramide. Taken together, these data support a crucial role for Ser(508) in the regulation of S-SMase secretion, and they suggest distinct metabolic roles for S-SMase and L-SMase.

Identification and characterization of eight novel SMPD1 mutations causing types A and B Niemann-Pick disease

Types A and B Niemann-Pick disease (NPD) result from the deficient activity of acid sphingomyelinase (ASM), due to mutations in the sphingomyelin phosphodiesterase 1 (SMPD1) gene. Here we report the identification, characterization and genotype/phenotype correlations of eight novel mutations in six unrelated NPD patients. These mutations included seven missense mutations: c.631T>C (p.W211R), c.757G>C (p.D253H), c.940G>A (p.V314M), c.1280A>G (p.H427R), c.1564A>G (p.N522S), c.1575G>C (p.Q525H) and c.1729A>G (p.H577R), and a novel frameshift mutation, c.1657delACCGCCT (fsT553). Each missense mutation was expressed in 293T or COS-7 cells; mutant enzymes p.W211R, p.D253H, p.H427R and p.H577R had <1% of expressed wild-type activity, whereas p.V314M, p.N522S and p.Q525H had 21.7%, 10.1% and 64% of expressed wild-type activity, respectively. The c.1564A>G mutation obliterated a known N-glycosylation site and its p.N522S mutant enzyme had ~10% of expressed wild-type activity. Western blot analysis revealed that each mutant protein was expressed at near wild-type amounts, despite their differences in residual activity. The novel seven-base deletion occurred at codon 553, leading to a premature truncation after residue 609. The expression studies predicted the clinical phenotypes of the six patients: two type A patients had genotypes with only type A alleles [c.631T>C (p.W211R), c.757G>C (p.D253H) and c.1729A>G (p.H577R)], and the other four type B disease patients had at least one neuroprotective mutant type B allele [c.940G>A (p.V314M), c.1280A>G (p.H427R), c.1564A>G (p.N522S) and c.1575G>C (p.Q525H)] that expressed >5% residual ASM activity. Thus, these new mutations provide novel genotype/phenotype correlations and further document the genetic heterogeneity in types A and B NPD.

Identification and characterization of SMPD1 mutations causing Niemann-Pick types A and B in Spanish patients

Niemann-Pick disease (NPD) types A/B are both caused by a deficiency of lysosomal acid sphingomyelinase and display autosomal recessive inheritance. These two types of the disease were described according to the presence (type A) or absence (type B) of neurological symptoms. We present a molecular analysis of 19 Spanish NPD A/B patients and two from Maghreb. Eight of the patients had type A and 13 had type B NPD. All mutant SMPD1 alleles were identified, including 17 different mutations, 10 of which were novel. The only frequent mutations in the 21 NPD patients were c.1823_1825delGCC (p.R608del) (38%) and c.1445C>A (p.A482E) (9%). Genotype-phenotype correlations were established for most of the mutations and, in particular, the p.R608del-type B association was confirmed. This mutation accounts for 61.5% of the mutant alleles in the type B subgroup of patients. Expression studies performed on six of the identified mutations confirmed them to be disease-causing due to their low enzyme activity. An allele with a mutation affecting a noncanonical donor splice site produced only aberrant mRNAs, corresponding to previously reported nonfunctional SMPD1 minor transcripts. This study is the first exhaustive mutational analysis of Spanish Niemann-Pick A/B disease patients.