Cell-autonomous death of cerebellar purkinje neurons with autophagy in Niemann-Pick type C disease

From bench to bedside: cutting-edge applications of base editing and prime editing in precision medicine

CRISPR-based gene editing technology theoretically allows for precise manipulation of any genetic target within living cells, achieving the desired sequence modifications. This revolutionary advancement has fundamentally transformed the field of biomedicine, offering immense clinical potential for treating and correcting genetic disorders. In the treatment of most genetic diseases, precise genome editing that avoids the generation of mixed editing byproducts is considered the ideal approach. This article reviews the current progress of base editors and prime editors, elaborating on specific examples of their applications in the therapeutic field, and highlights opportunities for improvement. Furthermore, we discuss the specific performance of these technologies in terms of safety and efficacy in clinical applications, and analyze the latest advancements and potential directions that could influence the future development of genome editing technologies. Our goal is to outline the clinical relevance of this rapidly evolving scientific field and preview a roadmap for successful DNA base editing therapies for the treatment of hereditary or idiopathic diseases.

Mutant induced neurons and humanized mice enable identification of Niemann-Pick type C1 proteostatic therapies

Therapeutics that rescue folding, trafficking, and function of disease-causing missense mutants are sought for a host of human diseases, but efforts to leverage model systems to test emerging strategies have met with limited success. Such is the case for Niemann-Pick type C1 disease, a lysosomal disorder characterized by impaired intracellular cholesterol trafficking, progressive neurodegeneration, and early death. NPC1, a multipass transmembrane glycoprotein, is synthesized in the endoplasmic reticulum and traffics to late endosomes/lysosomes, but this process is often disrupted in disease. We sought to identify small molecules that promote folding and enable lysosomal localization and functional recovery of mutant NPC1. We leveraged a panel of isogenic human induced neurons expressing distinct NPC1 missense mutations. We used this panel to rescreen compounds that were reported previously to correct NPC1 folding and trafficking. We established mo56-hydroxycholesterol (mo56Hc) as a potent pharmacological chaperone for several NPC1 mutants. Furthermore, we generated mice expressing human I1061T NPC1, a common mutation in patients. We demonstrated that this model exhibited disease phenotypes and recapitulated the protein trafficking defects, lipid storage, and response to mo56Hc exhibited by human cells expressing I1061T NPC1. These tools established a paradigm for testing and validation of proteostatic therapeutics as an important step toward the development of disease-modifying therapies.

Evaluation of the landscape of pharmacodynamic biomarkers in Niemann-Pick Disease Type C (NPC)

Niemann-Pick disease type C (NPC) is an autosomal recessive, progressive disorder resulting from variants in NPC1 or NPC2 that leads to the accumulation of cholesterol and other lipids in late endosomes and lysosomes. The clinical manifestations of the disease vary by age of onset, and severity is often characterized by neurological involvement. To date, no disease-modifying therapy has been approved by the United States Food and Drug Administration (FDA) and treatment is typically supportive. The lack of robust biomarkers contributes to challenges associated with disease monitoring and quantifying treatment response. In recent years, advancements in detection methods have facilitated the identification of biomarkers in plasma and cerebral spinal fluid from patients with NPC, namely calbindin D, neurofilament light chain, 24(S)hydroxycholesterol, cholestane-triol, trihydroxycholanic acid glycinate, amyloid-β, total and phosphorylated tau, and N-palmitoyl-O-phosphocholine-serine. These biomarkers have been used to support several clinical trials as pharmacodynamic endpoints. Despite the significant advancements in laboratory techniques, translation of those advancements has lagged, and it remains unclear which biomarkers correlate with disease severity and progression, or which biomarkers could inform treatment response. In this review, we assess the landscape of biomarkers currently proposed to guide disease monitoring or indicate treatment response in patients with NPC.

Differently increased volumes of multiple brain areas in Npc1 mutant mice following various drug treatments

Background

Niemann-Pick disease type C1 (NPC1, MIM 257220) is a heritable lysosomal storage disease characterized by a progressive neurological degeneration that causes disability and premature death. A murine model of Npc1-/- displays a rapidly progressing form of Npc1 disease, which is characterized by weight loss, ataxia, and increased cholesterol storage. Npc1-/- mice receiving a combined therapy (COMBI) of miglustat (MIGLU), the neurosteroid allopregnanolone (ALLO) and the cyclic oligosaccharide 2-hydroxypropyl-β-cyclodextrin (HPßCD) showed prevention of Purkinje cell loss, improved motor function and reduced intracellular lipid storage. Although therapy of Npc1-/- mice with COMBI, MIGLU or HPßCD resulted in the prevention of body weight loss, reduced total brain weight was not positively influenced.

Methods

In order to evaluate alterations of different brain areas caused by pharmacotherapy, fresh volumes (volumes calculated from the volumes determined from paraffin embedded brain slices) of various brain structures in sham- and drug-treated wild type and mutant mice were measured using stereological methods.

Results

In the wild type mice, the volumes of investigated brain areas were not significantly altered by either therapy. Compared with the respective wild types, fresh volumes of specific brain areas, which were significantly reduced in sham-treated Npc1-/- mice, partly increased after the pharmacotherapies in all treatment strategies; most pronounced differences were found in the CA1 area of the hippocampus and in olfactory structures.

Discussion

Volumes of brain areas of Npc1-/- mice were not specifically changed in terms of functionality after administering COMBI, MIGLU, or HPßCD. Measurements of fresh volumes of brain areas in Npc1-/- mice could monitor region-specific changes and response to drug treatment that correlated, in part, with behavioral improvements in this mouse model.

Increase in hnRNPA1 Expression Suffices to Kill Motor Neurons in Transgenic Rats

A dominant mutation in hnRNPA1 causes amyotrophic lateral sclerosis (ALS), but it is not known whether this mutation leads to motor neuron death through increased or decreased function. To elucidate the relationship between pathogenic hnRNPA1 mutation and its native function, we created novel transgenic rats that overexpressed wildtype rat hnRNPA1 exclusively in motor neurons. This targeted expression of wildtype hnRNPA1 caused severe motor neuron loss and subsequent denervation muscle atrophy in transgenic rats that recapitulated the characteristics of ALS. These findings demonstrate that the augmentation of hnRNPA1 expression suffices to trigger motor neuron degeneration and the manifestation of ALS-like phenotypes. It is reasonable to infer that an amplification of an as-yet undetermined hnRNPA1 function plays a pivotal role in the pathogenesis of familial ALS caused by pathogenic hnRNPA1 mutation.

The Npc2Gt(LST105)BygNya mouse signifies pathological changes comparable to human Niemann-Pick type C2 disease

INTRODUCTION

Niemann-Pick type C2 disease (NP-C2) is a fatal neurovisceral disorder caused by defects in the lysosomal cholesterol transporter protein NPC2. Consequently, cholesterol and other lipids accumulate within the lysosomes, causing a heterogeneous spectrum of clinical manifestations. Murine models are essential for increasing the understanding of the complex pathology of NP-C2. This study, therefore, aims to describe the neurovisceral pathology in the NPC2-deficient mouse model to evaluate its correlation to human NP-C2.

METHODS

Npc2-/- mice holding the LST105 mutation were used in the present study (Npc2Gt(LST105)BygNya). Body and organ weight and histopathological evaluations were carried out in six and 12-week-old Npc2-/- mice, with a special emphasis on neuropathology. The Purkinje cell (PC) marker calbindin, the astrocytic marker GFAP, and the microglia marker IBA1 were included to assess PC degeneration and neuroinflammation, respectively. In addition, the pathology of the liver, lungs, and spleen was assessed using hematoxylin and eosin staining.

RESULTS

Six weeks old pre-symptomatic Npc2-/- mice showed splenomegaly and obvious neuropathological changes, especially in the cerebellum, where initial PC loss and neuroinflammation were evident. The Npc2-/- mice developed neurological symptoms at eight weeks of age, severely progressing until the end-stage of the disease at 12 weeks. At the end-stage of the disease, Npc2-/- mice were characterized by growth retardation, tremor, cerebellar ataxia, splenomegaly, foam cell accumulation in the lungs, liver, and spleen, brain atrophy, pronounced PC degeneration, and severe neuroinflammation.

CONCLUSION

The Npc2Gt(LST105)BygNya mouse model resembles the pathology seen in NP-C2 patients and denotes a valuable model for increasing the understanding of the complex disease manifestation and is relevant for testing the efficacies of new treatment strategies.

The Cerebellum in Niemann-Pick C1 Disease: Mouse Versus Man

Selective neuronal vulnerability is common to most degenerative disorders, including Niemann-Pick C (NPC), a rare genetic disease with altered intracellular trafficking of cholesterol. Purkinje cell dysfunction and loss are responsible for cerebellar ataxia, which is among the prevailing neurological signs of the NPC disease. In this review, we focus on some questions that are still unresolved. First, we frame the cerebellar vulnerability in the context of the extended postnatal time length by which the development of this structure is completed in mammals. In line with this thought, the much later development of cerebellar symptoms in humans is due to the later development and/or maturation of the cerebellum. Hence, the occurrence of developmental events under a protracted condition of defective intracellular cholesterol mobilization hits the functional maturation of the various cell types generating the ground of increased vulnerability. This is particularly consistent with the high cholesterol demand required for cell proliferation, migration, differentiation, and synapse formation/remodeling. Other major questions we address are why the progression of Purkinje cells loss is always from the anterior to the posterior lobes and why cerebellar defects persist in the mouse model even when genetic manipulations can lead to nearly normal survival.

Species-specific differences in NPC1 protein trafficking govern therapeutic response in Niemann-Pick type C disease

The folding and trafficking of transmembrane glycoproteins are essential for cellular homeostasis and are compromised in many diseases. In Niemann-Pick type C disease, a lysosomal disorder characterized by impaired intracellular cholesterol trafficking, the transmembrane glycoprotein NPC1 misfolds due to disease-causing missense mutations. While mutant NPC1 has emerged as a robust target for proteostasis modulators, drug development efforts have been unsuccessful in mouse models. Here, we demonstrated unexpected differences in trafficking through the medial Golgi between mouse and human I1061T-NPC1, a common disease-causing mutant. We established that these distinctions are governed by differences in the NPC1 protein sequence rather than by variations in the endoplasmic reticulum-folding environment. Moreover, we demonstrated direct effects of mutant protein trafficking on the response to small molecules that modulate the endoplasmic reticulum-folding environment by affecting Ca++ concentration. Finally, we developed a panel of isogenic human NPC1 iNeurons expressing WT, I1061T-, and R934L-NPC1 and demonstrated their utility in testing these candidate therapeutics. Our findings identify important rules governing mutant NPC1's response to proteostatic modulators and highlight the importance of species- and mutation-specific responses for therapy development.

Metabolic Alteration Analysis of Steroid Hormones in Niemann-Pick Disease Type C Model Cell Using Liquid Chromatography/Tandem Mass Spectrometry

Niemann–Pick disease type C (NPC) is an autosomal recessive disease caused by a functional deficiency of cholesterol-transporting proteins in lysosomes, and exhibits various clinical symptoms. Since mitochondrial dysfunction in NPC has recently been reported, cholesterol catabolism to steroid hormones may consequently be impaired. In this study, we developed a comprehensive steroid hormone analysis method using liquid chromatography/tandem mass spectrometry (LC–MS/MS) and applied it to analyze changes in steroid hormone concentrations in NPC model cells. We investigated the analytical conditions for simultaneous LC–MS/MS analysis, which could be readily separated from each other and showed good reproducibility. The NPC phenotype was verified as an NPC model with mitochondrial abnormalities using filipin staining and organelle morphology observations. Steroid hormones in the cell suspension and cell culture medium were also analyzed. Steroid hormone analysis indicated that the levels of six steroid hormones were significantly decreased in the NPC model cell and culture medium compared to those in the wild-type cell and culture medium. These results indicate that some steroid hormones change during NPC pathophysiology and this change is accompanied by mitochondrial abnormalities.

Defective Cystinosin, Aberrant Autophagy−Endolysosome Pathways, and Storage Disease: Towards Assembling the Puzzle

Epithelial cells that form the kidney proximal tubule (PT) rely on an intertwined ecosystem of vesicular membrane trafficking pathways to ensure the reabsorption of essential nutrients—a key requisite for homeostasis. The endolysosome stands at the crossroads of this sophisticated network, internalizing molecules through endocytosis, sorting receptors and nutrient transporters, maintaining cellular quality control via autophagy, and toggling the balance between PT differentiation and cell proliferation. Dysregulation of such endolysosome-guided trafficking pathways might thus lead to a generalized dysfunction of PT cells, often causing chronic kidney disease and life-threatening complications. In this review, we highlight the biological functions of endolysosome-residing proteins from the perspectives of understanding—and potentially reversing—the pathophysiology of rare inherited diseases affecting the kidney PT. Using cystinosis as a paradigm of endolysosome disease causing PT dysfunction, we discuss how the endolysosome governs the homeostasis of specialized epithelial cells. This review also provides a critical analysis of the molecular mechanisms through which defects in autophagy pathways can contribute to PT dysfunction, and proposes potential interventions for affected tissues. These insights might ultimately accelerate the discovery and development of new therapeutics, not only for cystinosis, but also for other currently intractable endolysosome-related diseases, eventually transforming our ability to regulate homeostasis and health.

Impaired Retromer Function in Niemann-Pick Type C Disease Is Dependent on Intracellular Cholesterol Accumulation

Niemann-Pick type C disease (NPC) is a rare inherited neurodegenerative disorder characterized by an accumulation of intracellular cholesterol within late endosomes and lysosomes due to NPC1 or NPC2 dysfunction. In this work, we tested the hypothesis that retromer impairment may be involved in the pathogenesis of NPC and may contribute to increased amyloidogenic processing of APP and enhanced BACE1-mediated proteolysis observed in NPC disease. Using NPC1-null cells, primary mouse NPC1-deficient neurons and NPC1-deficient mice (BALB/cNctr-Npc1m1N), we show that retromer function is impaired in NPC. This is manifested by altered transport of the retromer core components Vps26, Vps35 and/or retromer receptor sorLA and by retromer accumulation in neuronal processes, such as within axonal swellings. Changes in retromer distribution in NPC1 mouse brains were observed already at the presymptomatic stage (at 4-weeks of age), indicating that the retromer defect occurs early in the course of NPC disease and may contribute to downstream pathological processes. Furthermore, we show that cholesterol depletion in NPC1-null cells and in NPC1 mouse brains reverts retromer dysfunction, suggesting that retromer impairment in NPC is mechanistically dependent on cholesterol accumulation. Thus, we characterized retromer dysfunction in NPC and propose that the rescue of retromer impairment may represent a novel therapeutic approach against NPC.

Disruption of morphogenic and growth pathways in lysosomal storage diseases

The lysosome achieved a new protagonism that highlights its multiple cellular functions, such as in the catabolism of complex substrates, nutrient sensing, and signaling pathways implicated in cell metabolism and growth. Lysosomal storage diseases (LSDs) cause lysosomal accumulation of substrates and deficiency in trafficking of macromolecules. The substrate accumulation can impact one or several pathways which contribute to cell damage. Autophagy impairment and immune response are widely studied, but less attention is paid to morphogenic and growth pathways and its impact on the pathophysiology of LSDs. Hedgehog pathway is affected with abnormal expression and changes in distribution of protein levels, and a reduced number and length of primary cilia. Moreover, growth pathways are identified with delay in reactivation of mTOR that deregulate termination of autophagy and reformation of lysosomes. Insulin resistance caused by changes in lipids rafts has been described in different LSDs. While the genetic and biochemical bases of deficient proteins in LSDs are well understood, the secondary molecular mechanisms that disrupt wider biological processes associated with LSDs are only now becoming clearer. Therefore, we explored how specific signaling pathways can be related to specific LSDs, showing that a system medicine approach could be a valuable tool for the better understanding of LSD pathogenesis. This article is categorized under: Metabolic Diseases > Molecular and Cellular Physiology.

In vivo somatic cell base editing and prime editing

Recent advances in genome editing technologies have magnified the prospect of single-dose cures for many genetic diseases. For most genetic disorders, precise DNA correction is anticipated to best treat patients. To install desired DNA changes with high precision, our laboratory developed base editors (BEs), which can correct the four most common single-base substitutions, and prime editors, which can install any substitution, insertion, and/or deletion over a stretch of dozens of base pairs. Compared to nuclease-dependent editing approaches that involve double-strand DNA breaks (DSBs) and often result in a large percentage of uncontrolled editing outcomes, such as mixtures of insertions and deletions (indels), larger deletions, and chromosomal rearrangements, base editors and prime editors often offer greater efficiency with fewer byproducts in slowly dividing or non-dividing cells, such as those that make up most of the cells in adult animals. Both viral and non-viral in vivo delivery methods have now been used to deliver base editors and prime editors in animal models, establishing that base editors and prime editors can serve as effective agents for in vivo therapeutic genome editing in animals. This review summarizes examples of in vivo somatic cell (post-natal) base editing and prime editing and prospects for future development.

Improved systemic AAV gene therapy with a neurotrophic capsid in Niemann-Pick disease type C1 mice

Niemann-Pick C1 disease (NPC1) is a rare, fatal neurodegenerative disease caused by mutations in NPC1, which encodes the lysosomal cholesterol transport protein NPC1. Disease pathology involves lysosomal accumulation of cholesterol and lipids, leading to neurological and visceral complications. Targeting the central nervous system (CNS) from systemic circulation complicates treatment of neurological diseases with gene transfer techniques. Selected and engineered capsids, for example, adeno-associated virus (AAV)-PHP.B facilitate peripheral-to-CNS transfer and hence greater CNS transduction than parental predecessors. We report that systemic delivery to Npc1 m1N/m1N mice using an AAV-PHP.B vector ubiquitously expressing NPC1 led to greater disease amelioration than an otherwise identical AAV9 vector. In addition, viral copy number and biodistribution of GFP-expressing reporters showed that AAV-PHP.B achieved more efficient, albeit variable, CNS transduction than AAV9 in Npc1 m1N/m1N mice. This variability was associated with segregation of two alleles of the putative AAV-PHP.B receptor Ly6a in Npc1 m1N/m1N mice. Our data suggest that robust improvements in NPC1 disease phenotypes occur even with modest CNS transduction and that improved neurotrophic capsids have the potential for superior NPC1 AAV gene therapy vectors.

Cerebellar Long Noncoding RNA Expression Profile in a Niemann-Pick C Disease Mouse Model

Niemann-Pick type C (NP-C) disease is a neurodegenerative lysosomal storage disorder primarily caused by mutations in NPC1. However, its pathogenesis remains poorly understood. While mounting evidence has demonstrated the involvement of long noncoding RNAs (lncRNAs) in the pathogenesis of neurodegenerative disorders, the lncRNA expression profile in NP-C has not been determined. Here, we used RNA-seq analysis to determine lncRNA and mRNA expression profiles of the cerebella of NPC1^−/− mice. We found that 272 lncRNAs and 856 mRNAs were significantly dysregulated in NPC1^−/− mice relative to controls (≥ 2.0-fold, p < 0.05). Quantitative real-time PCR (qRT‐PCR) was utilized to validate the expression of selected lncRNAs and mRNAs. Next, a lncRNA-mRNA coexpression network was employed to examine the potential roles of the differentially expressed (DE) lncRNAs. Functional analysis revealed that mRNAs coexpressed with lncRNAs are mainly linked to immune system–related processes and neuroinflammation. Moreover, knockdown of the lncRNA H19 ameliorated changes in ROS levels and cell viability and suppressed the lipopolysaccharide (LPS)–induced inflammatory response in vitro. Our findings indicate that dysregulated lncRNA expression patterns are associated with NP-C pathogenesis and offer insight into the development of novel therapeutics based on lncRNAs.

TDP-43 proteinopathy occurs independently of autophagic substrate accumulation and underlies nuclear defects in Niemann-Pick C disease

AIMS

Neuronal cytoplasmic inclusions of TAR-DNA binding protein of 43 kDa (TDP-43) are a pathological hallmark of diverse neurodegenerative disorders, yet the processes that mediate their formation and their functional significance remain incompletely understood. Both dysfunction in autophagy and neuroinflammation have been linked to TDP-43 mislocalisation. Here, we investigate TDP-43 proteinopathy in Niemann-Pick type C disease (NPC), an autosomal recessive lysosomal storage disease (LSD) distinguished by the accumulation of unesterified cholesterol within late endosomes and lysosomes. NPC is characterised by neurodegeneration, neuroinflammation and multifocal disruption of the autophagy pathway.

METHODS

We utilised immunohistochemistry, confocal microscopy, electron microscopy and biochemical and gene expression studies to characterise TDP-43 pathology and autophagic substrate accumulation in Npc1-deficient mice.

RESULTS

In the NPC brain, cytoplasmic TDP-43 mislocalisation was independent of autophagic substrate accumulation. These pathologies occurred in distinct neuronal subtypes, as brainstem cholinergic neurons were more susceptible to TDP-43 mislocalisation, whereas glutamatergic neurons exhibited hallmarks of autophagic dysfunction. Furthermore, TDP-43 mislocalisation did not co-localise with markers of stress granules or progress to ubiquitinated aggregates over months in vivo, indicating a stable, early stage in the aggregation process. Neither microgliosis nor neuroinflammation were sufficient to drive TDP-43 proteinopathy in the NPC brain. Notably, cytoplasmic TDP-43 co-localised with the nuclear import factor importin α, and TDP-43 mislocalised neurons demonstrated nuclear membrane abnormalities and disruption of nucleocytoplasmic transport.

CONCLUSION

Our findings highlight the relationship between LSDs and TDP-43 proteinopathy, define its functional importance in NPC by triggering nuclear dysfunction, and expand the spectrum of TDP-43 pathology in the diseased brain.

Lipid-mediated motor-adaptor sequestration impairs axonal lysosome delivery leading to autophagic stress and dystrophy in Niemann-Pick type C

Niemann-Pick disease type C (NPC) is a neurodegenerative lysosomal storage disorder characterized by lipid accumulation in endolysosomes. An early pathologic hallmark is axonal dystrophy occurring at presymptomatic stages in NPC mice. However, the mechanisms underlying this pathologic change remain obscure. Here, we demonstrate that endocytic-autophagic organelles accumulate in NPC dystrophic axons. Using super-resolution and live-neuron imaging, we reveal that elevated cholesterol on NPC lysosome membranes sequesters kinesin-1 and Arl8 independent of SKIP and Arl8-GTPase activity, resulting in impaired lysosome transport into axons, contributing to axonal autophagosome accumulation. Pharmacologic reduction of lysosomal membrane cholesterol with 2-hydroxypropyl-β-cyclodextrin (HPCD) or elevated Arl8b expression rescues lysosome transport, thereby reducing axonal autophagic stress and neuron death in NPC. These findings demonstrate a pathological mechanism by which altered membrane lipid composition impairs lysosome delivery into axons and provide biological insights into the translational application of HPCD in restoring axonal homeostasis at early stages of NPC disease.

npc2-Deficient Zebrafish Reproduce Neurological and Inflammatory Symptoms of Niemann-Pick Type C Disease

Niemann-Pick type C (NPC) disease is an autosomal recessive lysosomal storage disease that is caused by a mutation of the NPC1 or NPC2 gene, in which un-esterified cholesterol and sphingolipids accumulate mainly in the liver, spleen, and brain. Abnormal lysosomal storage leads to cell damage, neurological problems, and premature death. The time of onset and severity of symptoms of NPC disease are highly variable. The molecular mechanisms that are responsible for NPC disease pathology are far from being understood. The present study generated and characterized a zebrafish mutant that lacks Npc2 protein that may be useful for studies at the organismal, cellular, and molecular levels and both small-scale and high-throughput screens. Using CRISPR/Cas9 technology, we knocked out the zebrafish homolog of NPC2. Five-day-old npc2 mutants were morphologically indistinguishable from wildtype larvae. We found that live npc2^-/- larvae exhibited stronger Nile blue staining. The npc2^-/- larvae exhibited low mobility and a high anxiety-related response. These behavioral changes correlated with downregulation of the mcu (mitochondrial calcium uniporter) gene, ppp3ca (calcineurin) gene, and genes that are involved in myelination (mbp and mpz). Histological analysis of adult npc2^-/- zebrafish revealed that pathological changes in the nervous system, kidney, liver, and pancreas correlated with inflammatory responses (i.e., the upregulation of il1, nfκβ, and mpeg; i.e., hallmarks of NPC disease). These findings suggest that the npc2 mutant zebrafish may be a model of NPC disease.

Graphene Quantum Dots Alleviate Impaired Functions in Niemann-Pick Disease Type C in Vivo

While the neuropathological characteristics of Niemann-Pick disease type C (NPC) result in a fatal diagnosis, the development of clinically available therapeutic agent remains a challenge. Here we propose graphene quantum dots (GQDs) as a potential candidate for the impaired functions in NPC in vivo. In addition to the previous findings that GQDs exhibit negligible long-term toxicity and are capable of penetrating the blood-brain barrier, GQD treatment reduces the aggregation of cholesterol in the lysosome through expressed physical interactions. GQDs also promote autophagy and restore defective autophagic flux, which, in turn, decreases the atypical accumulation of autophagic vacuoles. More importantly, the injection of GQDs inhibits the loss of Purkinje cells in the cerebellum while also demonstrating reduced activation of microglia. The ability of GQDs to alleviate impaired functions in NPC proves the promise and potential of the use of GQDs toward resolving NPC and other related disorders.

Loss of NPC1 enhances phagocytic uptake and impairs lipid trafficking in microglia

Niemann-Pick type C disease is a rare neurodegenerative disorder mainly caused by mutations in NPC1, resulting in abnormal late endosomal/lysosomal lipid storage. Although microgliosis is a prominent pathological feature, direct consequences of NPC1 loss on microglial function remain not fully characterized. We discovered pathological proteomic signatures and phenotypes in NPC1-deficient murine models and demonstrate a cell autonomous function of NPC1 in microglia. Loss of NPC1 triggers enhanced phagocytic uptake and impaired myelin turnover in microglia that precede neuronal death. Npc1^−/− microglia feature a striking accumulation of multivesicular bodies and impaired trafficking of lipids to lysosomes while lysosomal degradation function remains preserved. Molecular and functional defects were also detected in blood-derived macrophages of NPC patients that provide a potential tool for monitoring disease. Our study underscores an essential cell autonomous role for NPC1 in immune cells and implies microglial therapeutic potential.

Niemann-Pick type C disease is a rare childhood neurodegenerative disorder predominantly caused by mutations in NPC1, resulting in abnormal late endosomal and lysosomal defects. Here the authors show that NPC1 disruption largely impairs microglial function.

Loss-of-function mutation of c-Ret causes cerebellar hypoplasia in mice with Hirschsprung disease and Down's syndrome

The c-RET proto-oncogene encodes a receptor-tyrosine kinase. Loss-of-function mutations of RET have been shown to be associated with Hirschsprung disease and Down's syndrome (HSCR-DS) in humans. DS is known to involve cerebellar hypoplasia, which is characterized by reduced cerebellar size. Despite the fact that c-Ret has been shown to be associated with HSCR-DS in humans and to be expressed in Purkinje cells (PCs) in experimental animals, there is limited information about the role of activity of c-Ret/c-RET kinase in cerebellar hypoplasia. We found that a loss-of-function mutation of c-Ret Y1062 in PCs causes cerebellar hypoplasia in c-Ret mutant mice. Wild-type mice had increased phosphorylation of c-Ret in PCs during postnatal development, while c-Ret mutant mice had postnatal hypoplasia of the cerebellum with immature neurite outgrowth in PCs and granule cells (GCs). c-Ret mutant mice also showed decreased numbers of glial fibers and mitogenic sonic hedgehog (Shh)-positive vesicles in the external germinal layer of PCs. c-Ret-mediated cerebellar hypoplasia was rescued by subcutaneous injection of a smoothened agonist (SAG) as well as by reduced expression of Patched1, a negative regulator for Shh. Our results suggest that the loss-of-function mutation of c-Ret Y1062 results in the development of cerebellar hypoplasia via impairment of the Shh-mediated development of GCs and glial fibers in mice with HSCR-DS.

Pluripotent Stem Cells for Disease Modeling and Drug Discovery in Niemann-Pick Type C1

The lysosomal storage disorders Niemann-Pick disease Type C1 (NPC1) and Type C2 (NPC2) are rare diseases caused by mutations in the NPC1 or NPC2 gene. Both NPC1 and NPC2 are proteins responsible for the exit of cholesterol from late endosomes and lysosomes (LE/LY). Consequently, mutations in one of the two proteins lead to the accumulation of unesterified cholesterol and glycosphingolipids in LE/LY, displaying a disease hallmark. A total of 95% of cases are due to a deficiency of NPC1 and only 5% are caused by NPC2 deficiency. Clinical manifestations include neurological symptoms and systemic symptoms, such as hepatosplenomegaly and pulmonary manifestations, the latter being particularly pronounced in NPC2 patients. NPC1 and NPC2 are rare diseases with the described neurovisceral clinical picture, but studies with human primary patient-derived neurons and hepatocytes are hardly feasible. Obviously, induced pluripotent stem cells (iPSCs) and their derivatives are an excellent alternative for indispensable studies with these affected cell types to study the multisystemic disease NPC1. Here, we present a review focusing on studies that have used iPSCs for disease modeling and drug discovery in NPC1 and draw a comparison to commonly used NPC1 models.

Understanding and Treating Niemann-Pick Type C Disease: Models Matter

Biomedical research aims to understand the molecular mechanisms causing human diseases and to develop curative therapies. So far, these goals have been achieved for a small fraction of diseases, limiting factors being the availability, validity, and use of experimental models. Niemann–Pick type C (NPC) is a prime example for a disease that lacks a curative therapy despite substantial breakthroughs. This rare, fatal, and autosomal-recessive disorder is caused by defects in NPC1 or NPC2. These ubiquitously expressed proteins help cholesterol exit from the endosomal–lysosomal system. The dysfunction of either causes an aberrant accumulation of lipids with patients presenting a large range of disease onset, neurovisceral symptoms, and life span. Here, we note general aspects of experimental models, we describe the line-up used for NPC-related research and therapy development, and we provide an outlook on future topics.

Fbxo2 mediates clearance of damaged lysosomes and modifies neurodegeneration in the Niemann-Pick C brain

A critical response to lysosomal membrane permeabilization (LMP) is the clearance of damaged lysosomes through a selective form of macroautophagy known as lysophagy. Although regulators of this process are emerging, whether organ- and cell-specific components contribute to the control of lysophagy remains incompletely understood. Here, we examined LMP and lysophagy in Niemann-Pick type C (NPC) disease, an autosomal recessive disorder characterized by the accumulation of unesterified cholesterol within late endosomes and lysosomes, leading to neurodegeneration and early death. We demonstrated that NPC human fibroblasts show enhanced sensitivity to lysosomal damage as a consequence of lipid storage. Moreover, we described a role for the glycan-binding F-box protein 2 (Fbxo2) in CNS lysophagy. Fbxo2 functions as a component of the S phase kinase-associated protein 1–cullin 1–F-box protein (SKP1-CUL1-SCF) ubiquitin ligase complex. Loss of Fbxo2 in mouse primary cortical cultures delayed clearance of damaged lysosomes and decreased viability after lysosomal damage. Moreover, Fbxo2 deficiency in a mouse model of NPC exacerbated deficits in motor function, enhanced neurodegeneration, and reduced survival. Collectively, our data identified a role for Fbxo2 in CNS lysophagy and establish its functional importance in NPC.

Glycan binding protein Fbxo2 regulates lysophagy in the brain, and its deficiency exacerbates neuronal deficits in a mouse model of Niemann-Pick type C disease.

Single Cell Transcriptome Analysis of Niemann-Pick Disease, Type C1 Cerebella

Niemann-Pick disease, type C1 (NPC1) is a lysosomal disease characterized by endolysosomal storage of unesterified cholesterol and decreased cellular cholesterol bioavailability. A cardinal symptom of NPC1 is cerebellar ataxia due to Purkinje neuron loss. To gain an understanding of the cerebellar neuropathology we obtained single cell transcriptome data from control (Npc1^+/+) and both three-week-old presymptomatic and seven-week-old symptomatic mutant (Npc1^-/-) mice. In seven-week-old Npc1^-/- mice, differential expression data was obtained for neuronal, glial, vascular, and myeloid cells. As anticipated, we observed microglial activation and increased expression of innate immunity genes. We also observed increased expression of innate immunity genes by other cerebellar cell types, including Purkinje neurons. Whereas neuroinflammation mediated by microglia may have both neuroprotective and neurotoxic components, the contribution of increased expression of these genes by non-immune cells to NPC1 pathology is not known. It is possible that dysregulated expression of innate immunity genes by non-immune cells is neurotoxic. We did not anticipate a general lack of transcriptomic changes in cells other than microglia from presymptomatic three-week-old Npc1^-/- mice. This observation suggests that microglia activation precedes neuronal dysfunction. The data presented in this paper will be useful for generating testable hypotheses related to disease progression and Purkinje neurons loss as well as providing insight into potential novel therapeutic interventions.

Targeting defective sphingosine kinase 1 in Niemann-Pick type C disease with an activator mitigates cholesterol accumulation

Niemann-Pick type C (NPC) disease is a lysosomal storage disorder arising from mutations in the cholesterol-trafficking protein NPC1 (95%) or NPC2 (5%). These mutations result in accumulation of low-density lipoprotein-derived cholesterol in late endosomes/lysosomes, disruption of endocytic trafficking, and stalled autophagic flux. Additionally, NPC disease results in sphingolipid accumulation, yet it is unique among the sphingolipidoses because of the absence of mutations in the enzymes responsible for sphingolipid degradation. In this work, we examined the cause for sphingosine and sphingolipid accumulation in multiple cellular models of NPC disease and observed that the activity of sphingosine kinase 1 (SphK1), one of the two isoenzymes that phosphorylate sphingoid bases, was markedly reduced in both NPC1 mutant and NPC1 knockout cells. Conversely, SphK1 inhibition with the isotype-specific inhibitor SK1-I in WT cells induced accumulation of cholesterol and reduced cholesterol esterification. Of note, a novel SphK1 activator (SK1-A) that we have characterized decreased sphingoid base and complex sphingolipid accumulation and ameliorated autophagic defects in both NPC1 mutant and NPC1 knockout cells. Remarkably, in these cells, SK1-A also reduced cholesterol accumulation and increased cholesterol ester formation. Our results indicate that a SphK1 activator rescues aberrant cholesterol and sphingolipid storage and trafficking in NPC1 mutant cells. These observations highlight a previously unknown link between SphK1 activity, NPC1, and cholesterol trafficking and metabolism.

Longitudinal MEMRI analysis of brain phenotypes in a mouse model of Niemann-Pick Type C disease

Niemann-Pick Type C (NPC) is a rare genetic disorder characterized by progressive cell death in various tissues, particularly in the cerebellar Purkinje cells, with no known cure. Mouse models for human NPC have been generated and characterized histologically, behaviorally, and using longitudinal magnetic resonance imaging (MRI). Previous imaging studies revealed significant brain volume differences between mutant and wild-type animals, but stopped short of making volumetric comparisons of the cerebellar sub-regions. In this study, we present longitudinal manganese-enhanced MRI (MEMRI) data from cohorts of wild-type, heterozygote carrier, and homozygote mutant NPC mice, as well as deformation-based morphometry (DBM) driven brain volume comparisons across genotypes, including the cerebellar cortex, white matter, and nuclei. We also present the first comparisons of MEMRI signal intensities, reflecting brain and cerebellum sub-regional Mn²⁺-uptake over time and across genotypes.

Mass spectrometry imaging and LC/MS reveal decreased cerebellar phosphoinositides in Niemann-Pick type C1-null mice

Niemann-Pick disease type C1 (NPC1) is a lipid storage disorder in which cholesterol and glycosphingolipids accumulate in late endosomal/lysosomal compartments because of mutations in the NPC1 gene. A hallmark of NPC1 is progressive neurodegeneration of the cerebellum as well as visceral organ damage; however, the mechanisms driving this disease pathology are not fully understood. Phosphoinositides are phospholipids that play distinct roles in signal transduction and vesicle trafficking. Here, we utilized a consensus spectra analysis of MS imaging data sets and orthogonal LC/MS analyses to evaluate the spatial distribution of phosphoinositides and quantify them in cerebellar tissue from Npc1-null mice. Our results suggest significant depletion of multiple phosphoinositide species, including PI, PIP, and PIP₂, in the cerebellum of the Npc1-null mice in both whole-tissue lysates and myelin-enriched fractions. Additionally, we observed altered levels of the regulatory enzyme phosphatidylinositol 4-kinase type 2α in Npc1-null mice. In contrast, the levels of related kinases, phosphatases, and transfer proteins were unaltered in the Npc1-null mouse model, as observed by Western blot analysis. Our discovery of phosphoinositide lipid biomarkers for NPC1 opens new perspectives on the pathophysiology underlying this fatal neurodegenerative disease.

Correction of Niemann-Pick type C1 trafficking and activity with the histone deacetylase inhibitor valproic acid

Niemann-Pick type C (NPC) disease is primarily caused by mutations in the NPC1 gene and is characterized by the accumulation of unesterified cholesterol and lipids in the late endosomal (LE) and lysosomal (Ly) compartments. The most prevalent disease-linked mutation is the I1061T variant of NPC1, which exhibits defective folding and trafficking from the endoplasmic reticulum to the LE/Ly compartments. We now show that the FDA-approved histone deacetylase inhibitor (HDACi) valproic acid (VPA) corrects the folding and trafficking defect associated with I1061T-NPC1 leading to restoration of cholesterol homeostasis, an effect that is largely driven by a reduction in HDAC7 expression. The VPA-mediated trafficking correction is in part associated with an increase in the acetylation of lysine residues in the cysteine-rich domain of NPC1. The HDACi-mediated correction is synergistically improved by combining it with the FDA-approved anti-malarial, chloroquine, a known lysosomotropic compound, which improved the stability of the LE/Ly-localized fraction of the I1061T variant. We posit that combining the activity of VPA, to modulate epigenetically the cellular acetylome, with chloroquine, to alter the lysosomal environment to favor stability of the trafficked I1061T variant protein can have a significant therapeutic benefit in patients carrying at least one copy of the I1061T variant of NPC1, the most common disease-associated mutation leading to NPC disease. Given its ability to cross the blood-brain barrier, we posit VPA provides a potential mechanism to improve the response to 2-hydroxypropyl-β-cyclodextrin, by restoring a functional NPC1 to the cholesterol managing compartment as an adjunct therapy.

Cerebellar Astrocytes: Much More Than Passive Bystanders In Ataxia Pathophysiology

Ataxia is a neurodegenerative syndrome, which can emerge as a major element of a disease or represent a symptom of more complex multisystemic disorders. It comprises several forms with a highly variegated etiology, mainly united by motor, balance, and speech impairments and, at the tissue level, by cerebellar atrophy and Purkinje cells degeneration. For this reason, the contribution of astrocytes to this disease has been largely overlooked in the past. Nevertheless, in the last few decades, growing evidences are pointing to cerebellar astrocytes as crucial players not only in the progression but also in the onset of distinct forms of ataxia. Although the current knowledge on this topic is very fragmentary and ataxia type-specific, the present review will attempt to provide a comprehensive view of astrocytes’ involvement across the distinct forms of this pathology. Here, it will be highlighted how, through consecutive stage-specific mechanisms, astrocytes can lead to non-cell autonomous neurodegeneration and, consequently, to the behavioral impairments typical of this disease. In light of that, treating astrocytes to heal neurons will be discussed as a potential complementary therapeutic approach for ataxic patients, a crucial point provided the absence of conclusive treatments for this disease.

Cytosine and adenine base editing of the brain, liver, retina, heart and skeletal muscle of mice via adeno-associated viruses

The success of base editors for the study and treatment of genetic diseases depends on the ability to deliver them in vivo to the relevant cell types. Delivery via adeno-associated viruses (AAVs) is limited by AAV packaging capacity, which precludes the use of full-length base editors. Here, we report the application of dual AAVs for the delivery of split cytosine and adenine base editors that are then reconstituted by trans-splicing inteins. Optimized dual AAVs enable in vivo base editing at therapeutically relevant efficiencies and dosages in the mouse brain (up to 59% of unsorted cortical tissue), liver (38%), retina (38%), heart (20%) and skeletal muscle (9%). We also show that base editing corrects, in mouse brain tissue, a mutation that causes Niemann-Pick disease type C (a neurodegenerative ataxia), slowing down neurodegeneration and increasing lifespan. The optimized delivery vectors should facilitate the efficient introduction of targeted point mutations into multiple tissues of therapeutic interest.

Cytosine and adenine base editing of the brain, liver, retina, heart and skeletal muscle of mice via adeno-associated viruses

The success of base editors for the study and treatment of genetic diseases depends on the ability to deliver them in vivo to the relevant cell types. Delivery via adeno-associated viruses (AAVs) is limited by AAV packaging capacity, which precludes the use of full-length base editors. Here, we report the application of dual AAVs for the delivery of split cytosine and adenine base editors that are then reconstituted by trans-splicing inteins. Optimized dual AAVs enable in vivo base editing at therapeutically relevant efficiencies and dosages in the mouse brain (up to 59% of unsorted cortical tissue), liver (38%), retina (38%), heart (20%) and skeletal muscle (9%). We also show that base editing corrects, in mouse brain tissue, a mutation that causes Niemann-Pick disease type C (a neurodegenerative ataxia), slowing down neurodegeneration and increasing lifespan. The optimized delivery vectors should facilitate the efficient introduction of targeted point mutations into multiple tissues of therapeutic interest.

The TMEM106B FTLD-protective variant, rs1990621, is also associated with increased neuronal proportion

Apart from amyloid β deposition and tau neurofibrillary tangles, Alzheimer's disease (AD) is a neurodegenerative disorder characterized by neuronal loss and astrocytosis in the cerebral cortex. The goal of this study is to investigate genetic factors associated with the neuronal proportion in health and disease. To identify cell-autonomous genetic variants associated with neuronal proportion in cortical tissues, we inferred cellular population structure from bulk RNA-Seq derived from 1536 individuals. We identified the variant rs1990621 located in the TMEM106B gene region as significantly associated with neuronal proportion (p value = 6.40 × 10-07) and replicated this finding in an independent dataset (p value = 7.41 × 10-04) surpassing the genome-wide threshold in the meta-analysis (p value = 9.42 × 10-09). This variant is in high LD with the TMEM106B non-synonymous variant p.T185S (rs3173615; r2 = 0.98) which was previously identified as a protective variant for frontotemporal lobar degeneration (FTLD). We stratified the samples by disease status, and discovered that this variant modulates neuronal proportion not only in AD cases, but also several neurodegenerative diseases and in elderly cognitively healthy controls. Furthermore, we did not find a significant association in younger controls or schizophrenia patients, suggesting that this variant might increase neuronal survival or confer resilience to the neurodegenerative process. The single variant and gene-based analyses also identified an overall genetic association between neuronal proportion, AD and FTLD risk. These results suggest that common pathways are implicated in these neurodegenerative diseases, that implicate neuronal survival. In summary, we identified a protective variant in the TMEM106B gene that may have a neuronal protection effect against general aging, independent of disease status, which could help elucidate the relationship between aging and neuronal survival in the presence or absence of neurodegenerative disorders. Our findings suggest that TMEM106B could be a potential target for neuronal protection therapies to ameliorate cognitive and functional deficits.

Identification of Novel Pathways Associated with Patterned Cerebellar Purkinje Neuron Degeneration in Niemann-Pick Disease, Type C1

Niemann-Pick disease, type C1 (NPC1) is a lysosomal disease characterized by progressive cerebellar ataxia. In NPC1, a defect in cholesterol transport leads to endolysosomal storage of cholesterol and decreased cholesterol bioavailability. Purkinje neurons are sensitive to the loss of NPC1 function. However, degeneration of Purkinje neurons is not uniform. They are typically lost in an anterior-to-posterior gradient with neurons in lobule X being resistant to neurodegeneration. To gain mechanistic insight into factors that protect or potentiate Purkinje neuron loss, we compared RNA expression in cerebellar lobules III, VI, and X from control and mutant mice. An unexpected finding was that the gene expression differences between lobules III/VI and X were more pronounced than those observed between mutant and control mice. Functional analysis of genes with anterior to posterior gene expression differences revealed an enrichment of genes related to neuronal cell survival within the posterior cerebellum. This finding is consistent with the observation, in multiple diseases, that posterior Purkinje neurons are, in general, resistant to neurodegeneration. To our knowledge, this is the first study to evaluate anterior to posterior transcriptome-wide changes in gene expression in the cerebellum. Our data can be used to not only explore potential pathological mechanisms in NPC1, but also to further understand cerebellar biology.

Niemann-Pick type C disease: cellular pathology and pharmacotherapy

Niemann-Pick type C disease (NPCD) was first described in 1914 and affects approximately 1 in 150 000 live births. It is characterized clinically by diverse symptoms affecting liver, spleen, motor control, and brain; premature death invariably results. Its molecular origins were traced, as late as 1997, to a protein of late endosomes and lysosomes which was named NPC1. Mutation or absence of this protein leads to accumulation of cholesterol in these organelles. In this review, we focus on the intracellular events that drive the pathology of this disease. We first introduce endocytosis, a much-studied area of dysfunction in NPCD cells, and survey the various ways in which this process malfunctions. We briefly consider autophagy before attempting to map the more complex pathways by which lysosomal cholesterol storage leads to protein misregulation, mitochondrial dysfunction, and cell death. We then briefly introduce the metabolic pathways of sphingolipids (as these emerge as key species for treatment) and critically examine the various treatment approaches that have been attempted to date.

The intersection of lysosomal and endoplasmic reticulum calcium with autophagy defects in lysosomal diseases

The lysosomal storage disorders (LSDs) encompass a group of more than 50 inherited diseases characterized by the accumulation of lysosomal substrates. Two-thirds of patients experience significant neurological symptoms, but the mechanisms of neurodegeneration are not well understood. Interestingly, a wide range of LSDs show defects in both autophagy and Ca2+ homeostasis, which is notable as Ca2+ is a key regulator of autophagy. The crosstalk between these pathways in the context of LSD pathogenesis is not well characterized, but further understanding of this relationship could open up promising therapeutic targets. This review discusses the role of endoplasmic reticulum and lysosomal Ca2+ in autophagy regulation and highlights what is known about defects in autophagy and Ca2+ homeostasis in two LSDs, Niemann-Pick type C disease and Gaucher disease.

LC3 Immunostaining in the Inferior Olivary Nuclei of Cats With Niemann-Pick Disease Type C1 Is Associated With Patterned Purkinje Cell Loss

The feline model of Niemann-Pick disease, type C1 (NPC1) recapitulates the clinical, neuropathological, and biochemical abnormalities present in children with NPC1. The hallmarks of disease are the lysosomal storage of unesterified cholesterol and multiple sphingolipids in neurons, and the spatial and temporal distribution of Purkinje cell death. In feline NPC1 brain, microtubule-associated protein 1 light chain 3 (LC3) accumulations, indicating autophagosomes, were found within axons and presynaptic terminals. High densities of accumulated LC3 were seen in subdivisions of the inferior olive, which project to cerebellar regions that show the most Purkinje cell loss, suggesting that autophagic abnormalities in specific climbing fibers may contribute to the spatial pattern of Purkinje cell loss seen. Biweekly intrathecal administration of 2-hydroxypropyl-beta cyclodextrin (HPβCD) ameliorated neurological dysfunction, reduced cholesterol and sphingolipid accumulation, and increased lifespan in NPC1 cats. LC3 pathology was reduced in treated animals suggesting that HPβCD administration also ameliorates autophagic abnormalities. This study is the first to (i) identify specific brain regions exhibiting autophagic abnormalities in any species with NPC1, (ii) provide evidence of differential vulnerability among discrete brain nuclei and pathways, and (iii) show the amelioration of these abnormalities in NPC1 cats treated with HPβCD.

Autonomous Purkinje cell axonal dystrophy causes ataxia in peroxisomal multifunctional protein-2 deficiency

Peroxisomes play a crucial role in normal neurodevelopment and in the maintenance of the adult brain. This depends largely on intact peroxisomal β-oxidation given the similarities in pathologies between peroxisome biogenesis disorders and deficiency of multifunctional protein-2 (MFP2), the central enzyme of this pathway. Recently, adult patients diagnosed with cerebellar ataxia were shown to have mild mutations in the MFP2 gene, hydroxy-steroid dehydrogenase (17 beta) type 4 (HSD17B4). Cerebellar atrophy also develops in MFP2 deficient mice but the cellular origin of the degeneration is unexplored. In order to investigate whether peroxisomal β-oxidation is essential within Purkinje cells, the sole output neurons of the cerebellum, we generated and characterized a mouse model with Purkinje cell selective deletion of the MFP2 gene. We show that selective loss of MFP2 from mature cerebellar Purkinje neurons causes a late-onset motor phenotype and progressive Purkinje cell degeneration, thereby mimicking ataxia and cerebellar deterioration in patients with mild HSD17B4 mutations. We demonstrate that swellings on Purkinje cell axons coincide with ataxic behavior and precede neurodegeneration. Loss of Purkinje cells occurs in a characteristic banded pattern, proceeds in an anterior to posterior fashion and is accompanied by progressive astro- and microgliosis. These data prove that the peroxisomal β-oxidation pathway is required within Purkinje neurons to maintain their axonal integrity, independent of glial dysfunction.

L-leucine and SPNS1 coordinately ameliorate dysfunction of autophagy in mouse and human Niemann-Pick type C disease

Lysosomal storage disorders are characterized by progressive accumulation of undigested macromolecules within the cell due to lysosomal dysfunction. 573C10 is a Schwann cell line derived from a mouse model of Niemann-Pick type C disease-1, NPC (−/−). Under serum-starved conditions, NPC (−/−) cells manifested impaired autophagy accompanied by an increase in the amount of p62 and lysosome enlargement. Addition of L-leucine to serum-starved NPC (−/−) cells ameliorated the enlargement of lysosomes and the p62 accumulation. Similar autophagy defects were observed in NPC (−/−) cells even without serum starvation upon the knockdown of Spinster-like 1 (SPNS1), a putative transporter protein thought to function in lysosomal recycling. Conversely, SPNS1 overexpression impeded the enlargement of lysosomes, p62 accumulation and mislocalization of the phosphorylated form of the mechanistic Target of rapamycin in NPC (−/−) cells. In addition, we found a reduction in endogenous SPNS1 expression in fibroblasts derived from NPC-1 patients compared with normal fibroblasts. We propose that SPNS1-dependent L-leucine export across the lysosomal membrane is a key step for triggering autophagy, and that this mechanism is impaired in NPC-1.

Systemic AAV9 gene therapy improves the lifespan of mice with Niemann-Pick disease, type C1

Niemann-Pick disease, type C1 (NPC1) is a heritable lysosomal storage disease characterized by a progressive neurological degeneration that causes disability and premature death. A murine model of NPC1 disease (Npc1-/-) displays a rapidly progressing form of NPC1 disease which is characterized by weight loss, ataxia, increased cholesterol storage, loss of cerebellar Purkinje neurons and early lethality. To test the potential efficacy of gene therapy for NPC1, we constructed adeno-associated virus serotype 9 (AAV9) vectors to deliver the NPC1 gene under the transcriptional control of the neuronal-specific (CamKII) or a ubiquitous (EF1a) promoter. The Npc1-/- mice that received a single dose of AAV9-CamKII-NPC1 as neonates (2.6 × 1011GC) or at weaning (1.3 × 1012GC), and the mice that received a single dose of AAV9-EF1a-NPC1 at weaning (1.2 × 1012GC), exhibited an increased life span, characterized by delayed weight loss and diminished motor decline. Cholesterol storage and Purkinje neuron loss were also reduced in the central nervous system of AAV9 treated Npc1-/- mice. Treatment with AAV9-EF1a-NPC1, as compared to AAV9-CamKII-NPC1, resulted in significantly increased survival (mean survival increased from 69 days to 166 and 97 days, respectively) and growth, and reduced hepatic-cholesterol accumulation. Our results provide the first demonstration that gene therapy may represent a therapeutic option for NPC1 patients and suggest that extraneuronal NPC1 expression can further augment the lifespan of the Npc1-/- mice after systemic AAV gene delivery.

N-butyldeoxynojirimycin delays motor deficits, cerebellar microgliosis, and Purkinje cell loss in a mouse model of mucolipidosis type IV

Mucolipidosis type IV (MLIV) is a lysosomal storage disease exhibiting progressive intellectual disability, motor impairment, and premature death. There is currently no cure or corrective treatment. The disease results from mutations in the gene encoding mucolipin-1, a transient receptor potential channel believed to play a key role in lysosomal calcium egress. Loss of mucolipin-1 and subsequent defects lead to a host of cellular aberrations, including accumulation of glycosphingolipids (GSLs) in neurons and other cell types, microgliosis and, as reported here, cerebellar Purkinje cell loss. Several studies have demonstrated that N-butyldeoxynojirimycin (NB-DNJ, also known as miglustat), an inhibitor of the enzyme glucosylceramide synthase (GCS), successfully delays the onset of motor deficits, improves longevity, and rescues some of the cerebellar abnormalities (e.g., Purkinje cell death) seen in another lysosomal disease known as Niemann-Pick type C (NPC). Given the similarities in pathology between MLIV and NPC, we examined whether miglustat would be efficacious in ameliorating disease progression in MLIV. Using a full mucolipin-1 knockout mouse (Mcoln1-/-), we found that early miglustat treatment delays the onset and progression of motor deficits, delays cerebellar Purkinje cell loss, and reduces cerebellar microgliosis characteristic of MLIV disease. Quantitative mass spectrometry analyses provided new data on the GSL profiles of murine MLIV brain tissue and showed that miglustat partially restored the wild type profile of white matter enriched lipids. Collectively, our findings indicate that early miglustat treatment delays the progression of clinically relevant pathology in an MLIV mouse model, and therefore supports consideration of miglustat as a therapeutic agent for MLIV disease in humans.

Activation of PKC triggers rescue of NPC1 patient specific iPSC derived glial cells from gliosis

BACKGROUND

Niemann-Pick disease Type C1 (NPC1) is a rare progressive neurodegenerative disorder caused by mutations in the NPC1 gene. The pathological mechanisms, underlying NPC1 are not yet completely understood. Especially the contribution of glial cells and gliosis to the progression of NPC1, are controversially discussed. As an analysis of affected cells is unfeasible in NPC1-patients, we recently developed an in vitro model system, based on cells derived from NPC1-patient specific iPSCs. Here, we asked if this model system recapitulates gliosis, observed in non-human model systems and NPC1 patient post mortem biopsies. We determined the amount of reactive astrocytes and the regulation of the intermediate filaments GFAP and vimentin, all indicating gliosis. Furthermore, we were interested in the assembly and phosphorylation of these intermediate filaments and finally the impact of the activation of protein kinase C (PKC), which is described to ameliorate the pathogenic phenotype of NPC1-deficient fibroblasts, including hypo-phosphorylation of vimentin and cholesterol accumulation.

METHODS

We analysed glial cells derived from NPC1 patient specific induced pluripotent stem cells, carrying different NPC1 mutations. The amount of reactive astrocytes was determined by means of immuncytochemical stainings and FACS-analysis. Semi-quantitative western blot was used to determine the amount of phosphorylated GFAP and vimentin. Cholesterol accumulation was analysed by Filipin staining and quantified by Amplex Red Assay. U18666A was used to induce NPC1 phenotype in unaffected cells of the control cell line. Phorbol 12-myristate 13-acetate (PMA) was used to activate PKC.

RESULTS

Immunocytochemical detection of GFAP, vimentin and Ki67 revealed that NPC1 mutant glial cells undergo gliosis. We found hypo-phosphorylation of the intermediate filaments GFAP and vimentin and alterations in the assembly of these intermediate filaments in NPC1 mutant cells. The application of U18666A induced not only NPC1 phenotypical accumulation of cholesterol, but characteristics of gliosis in glial cells derived from unaffected control cells. The application of phorbol 12-myristate 13-acetate, an activator of protein kinase C resulted in a significantly reduced number of reactive astrocytes and further characteristics of gliosis in NPC1-deficient cells. Furthermore, it triggered a restoration of cholesterol amounts to level of control cells.

CONCLUSION

Our data demonstrate that glial cells derived from NPC1-patient specific iPSCs undergo gliosis. The application of U18666A induced comparable characteristics in un-affected control cells, suggesting that gliosis is triggered by hampered function of NPC1 protein. The activation of protein kinase C induced an amelioration of gliosis, as well as a reduction of cholesterol amount. These results provide further support for the line of evidence that gliosis might not be only a secondary reaction to the loss of neurons, but might be a direct consequence of a reduced PKC activity due to the phenotypical cholesterol accumulation observed in NPC1. In addition, our data support the involvement of PKCs in NPC1 disease pathogenesis and suggest that PKCs may be targeted in future efforts to develop therapeutics for NPC1 disease.

Decreased calcium flux in Niemann-Pick type C1 patient-specific iPSC-derived neurons due to higher amount of calcium-impermeable AMPA receptors

Niemann-Pick disease type C1 (NPC1) is a rare progressive neurodegenerative disorder caused by mutations in the NPC1 gene, resulting mainly in the accumulation of cholesterol and the ganglioside GM2. Recently, we described accumulations of these lipids in neuronal differentiated cells derived from NPC1 patient-specific induced pluripotent stem cells (iPSCs). As these lipids are essential for proper cell membrane composition, we were interested in the expression and function of voltage-gated ion channels and excitatory AMPA receptors (AMPARs) in neurons derived from three patient-specific iPSC lines. By means of patch clamp recordings and microfluorimetric measurements of calcium (Ca²⁺), we examined the expression of voltage-gated ion channels and AMPARs. Cells of the three used cell lines carrying the c.1836A>C/c.1628delC, the c.1180T>C or the c.3182T>C mutation demonstrated a significantly reduced AMPA-induced Ca²⁺-influx, suggesting an altered expression profile of these receptors. RT-qPCR revealed a significant upregulation of mRNA for the AMPA receptor subunits GluA1 and GluA2 and western blot analysis showed increased protein level of GluA2. Thus, we conclude that the observed reduced Ca²⁺-influx is based on an increase of GluA2 containing Ca²⁺-impermeable AMPARs. An attenuated function of GluRs in neurons potentially contributes to the progressive neurodegeneration observed in NPC1 and might represent an objective in regard of the development of new therapeutic approaches in NPC1.

Methyl-β-cyclodextrin restores impaired autophagy flux in Niemann-Pick C1-deficient cells through activation of AMPK

The drug 2-hydroxypropyl-β-cyclodextrin (HPβCD) reduces lysosomal cholesterol accumulation in Niemann-Pick disease, type C (NPC) and has been advanced to human clinical trials. However, its mechanism of action for reducing cholesterol accumulation in NPC cells is uncertain and its molecular target is unknown. We found that methyl-β-cyclodextrin (MβCD), a potent analog of HPβCD, restored impaired macroautophagy/autophagy flux in Niemann-Pick disease, type C1 (NPC1) cells. This effect was mediated by a direct activation of AMP-activated protein kinase (AMPK), an upstream kinase in the autophagy pathway, through MβCD binding to its β-subunits. Knockdown of PRKAB1 or PRKAB2 (encoding the AMPK β1 or β2 subunit) expression and an AMPK inhibitor abolished MβCD-mediated reduction of cholesterol storage in NPC1 cells. The results demonstrate that AMPK is the molecular target of MβCD and its activation enhances autophagy flux, thereby mitigating cholesterol accumulation in NPC1 cells. The results identify AMPK as an attractive target for drug development to treat NPC.

AAV9-NPC1 significantly ameliorates Purkinje cell death and behavioral abnormalities in mouse NPC disease

Niemann-Pick type C (NPC) disease is a fatal inherited neurodegenerative disorder caused by loss-of-function mutations in the NPC1 or NPC2 gene. There is no effective way to treat NPC disease. In this study, we used adeno-associated virus (AAV) serotype 9 (AAV9) to deliver a functional NPC1 gene systemically into NPC1-/- mice at postnatal day 4. One single AAV9-NPC1 injection resulted in robust NPC1 expression in various tissues, including brain, heart, and lung. Strikingly, AAV9-mediated NPC1 delivery significantly promoted Purkinje cell survival, restored locomotor activity and coordination, and increased the lifespan of NPC1-/- mice. Our work suggests that AAV-based gene therapy is a promising means to treat NPC disease.

Modeling Niemann Pick type C1 using human embryonic and induced pluripotent stem cells

Data generated in Niemann Pick type C1 (NPC1) human embryonic and human induced pluripotent stem cell derived neurons complement on-going studies in animal models and provide the first example, in disease-relevant human cells, of processes that underlie preferential neuronal defects in a NPC1. Our work and that of other investigators in human neurons derived from stem cells highlight the importance of performing rigorous mechanistic studies in relevant cell types to guide drug discovery and therapeutic development, alongside of existing animal models. Through the use of human stem cell-derived models of disease, we can identify and discover or repurpose drugs that revert early events that lead to neuronal failure in NPC1. Together with the study of disease pathogenesis and efficacy of therapies in animal models, these strategies will fulfill the promise of stem cell technology in the development of new treatments for human diseases. This article is part of a Special Issue entitled SI: Exploiting human neurons.

Toll-interacting protein deficiency promotes neurodegeneration via impeding autophagy completion in high-fat diet-fed ApoE-/- mouse model

The excessive accumulation of specific cellular proteins or autophagic vacuoles (AVs) within neurons is a pathologic hallmark of neurodegenerative diseases. Constitutive autophagy in neurons prevents abnormal intracellular protein aggregation and is critical for maintaining cell survival. Since our previous study showed that Toll-interacting protein (Tollip)-deficient macrophages had constitutive disruption of endosome-lysosome fusion, we hypothesize that Tollip deficiency may also promote neuron death via blockage of autophagy completion. Indeed, we observed significantly higher levels of neuron death in the brain regions of cerebral cortex, hippocampus, and cerebellum from ApoE^-/-/Tollip^-/- mice as compared to ApoE^-/- mice fed with high fat diet (HFD). We further documented diminished density of neurons and increased ratios of TUNEL positive cells in the hippocampus of ApoE^-/-/Tollip^-/- mice. The ultrastructural electron microscopy analyses revealed neuron cell shrinkage as well as loss of intracellular structure in brain tissues from ApoE^-/-/Tollip^-/- mice. There was dramatic accumulation of autophagosomes in the cytoplasm, elevated accumulation of β-amyloid and α-synuclein, and increased levels of p62 and Parkin in the brain tissues from ApoE^-/-/Tollip^-/- mice as compared to ApoE^-/- mice. Our data suggest that Tollip may play a crucial role in sustaining neuron health by facilitating the completion of autophagy, and that Tollip-deficiency may accelerate neuron death related to neurological disease such as Alzheimer's disease.

Differential Transcriptome Networks between IDO1-Knockout and Wild-Type Mice in Brain Microglia and Macrophages

Microglia in the brain and macrophages in peripheral organs are cell types responsible for immune response to challenges. Indoleamine 2,3-dioxygenase 1 (IDO1) is an immunomodulatory enzyme of the tryptophan pathway that is expressed in the brain. The higher activity of IDO1 in response to immune challenge has been implicated in behavioral disorders. The impact of IDO1 depletion on the microglia transcriptome has not been studied. An investigation of the transcript networks in the brain microglia from IDO1-knockout (IDO1-KO) mice was undertaken, relative to peripheral macrophages and to wild-type (WT) mice under unchallenged conditions. Over 105 transcript isoforms were differentially expressed between WT and IDO1-KO within cell type. Within microglia, Saa3 and Irg1 were over-expressed in IDO1-KO relative to WT. Within macrophages, Csf3 and Sele were over-expressed in IDO1-KO relative to WT. Among the genes differentially expressed between strains, enriched biological processes included ion homeostasis and ensheathment of neurons within microglia, and cytokine and chemokine expression within macrophages. Over 11,110 transcript isoforms were differentially expressed between microglia and macrophages and of these, over 10,800 transcripts overlapped between strains. Enriched biological processes among the genes over- and under-expressed in microglia relative to macrophages included cell adhesion and apoptosis, respectively. Detected only in microglia or macrophages were 421 and 43 transcript isoforms, respectively. Alternative splicing between cell types based on differential transcript isoform abundance was detected in 210 genes including Phf11d, H2afy, and Abr. Across strains, networks depicted a predominance of genes under-expressed in microglia relative to macrophages that may be a precursor for the different response of both cell types to challenges. The detected transcriptome differences enhance the understanding of the role of IDO1 in the microglia transcriptome under unchallenged conditions.

Heat Shock Protein Beta-1 Modifies Anterior to Posterior Purkinje Cell Vulnerability in a Mouse Model of Niemann-Pick Type C Disease

Selective neuronal vulnerability is characteristic of most degenerative disorders of the CNS, yet mechanisms underlying this phenomenon remain poorly characterized. Many forms of cerebellar degeneration exhibit an anterior-to-posterior gradient of Purkinje cell loss including Niemann-Pick type C1 (NPC) disease, a lysosomal storage disorder characterized by progressive neurological deficits that often begin in childhood. Here, we sought to identify candidate genes underlying vulnerability of Purkinje cells in anterior cerebellar lobules using data freely available in the Allen Brain Atlas. This approach led to the identification of 16 candidate neuroprotective or susceptibility genes. We demonstrate that one candidate gene, heat shock protein beta-1 (HSPB1), promoted neuronal survival in cellular models of NPC disease through a mechanism that involved inhibition of apoptosis. Additionally, we show that over-expression of wild type HSPB1 or a phosphomimetic mutant in NPC mice slowed the progression of motor impairment and diminished cerebellar Purkinje cell loss. We confirmed the modulatory effect of Hspb1 on Purkinje cell degeneration in vivo, as knockdown by Hspb1 shRNA significantly enhanced neuron loss. These results suggest that strategies to promote HSPB1 activity may slow the rate of cerebellar degeneration in NPC disease and highlight the use of bioinformatics tools to uncover pathways leading to neuronal protection in neurodegenerative disorders.

Niemann-Pick type C1 (NPC) disease is an autosomal recessive lipid storage disorder for which there is no effective treatment. Patients develop a clinically heterogeneous phenotype that typically includes childhood onset neurodegeneration and early death. Mice with loss of function mutations in the Npc1 gene model many aspects of the human disease, including cerebellar degeneration that results in marked ataxia. Cerebellar Purkinje cells in mutant mice exhibit striking selective vulnerability, with neuron loss in anterior lobules and preservation in posterior lobules. As this anterior to posterior gradient is reproduced following cell autonomous deletion of Npc1 and is also observed in other forms of cerebellar degeneration, we hypothesized that it is mediated by differential gene expression. To test this notion, we probed the Allen Brain Atlas to identify 16 candidate neuroprotective or susceptibility genes. We confirmed that one of these genes, encoding the small heat shock protein Hspb1, promotes survival in cell culture models of NPC disease. Moreover, we found that modulating Hspb1 expression in NPC mice promoted (following over-expression) or diminished (following knock-down) Purkinje cell survival, confirming its neuroprotective activity. We suggest that this approach may be similarly used in other diseases to uncover pathways that modify selective neuronal vulnerability.