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

Cerebellar Heterogeneity and Selective vulnerability in Spinocerebellar Ataxia Type 1 (SCA1)

Heterogeneity is one of the key features of the healthy brain and selective vulnerability characterizes many, if not all, neurodegenerative diseases. While cerebellum contains majority of brain cells, neither its heterogeneity nor selective vulnerability in disease are well understood. Here we describe molecular, cellular and functional heterogeneity in the context of healthy cerebellum as well as in cerebellar disease Spinocerebellar Ataxia Type 1 (SCA1). We first compared disease pathology in cerebellar vermis and hemispheres across anterior to posterior axis in a knock-in SCA1 mouse model. Using immunohistochemistry, we demonstrated earlier and more severe pathology of PCs and glia in the posterior cerebellar vermis of SCA1 mice. We also demonstrate heterogeneity of Bergmann glia in the unaffected, wild-type mice. Then, using RNA sequencing, we found both shared, as well as, posterior cerebellum-specific molecular mechanisms of pathogenesis that include exacerbated gene dysregulation, increased number of altered signaling pathways, and decreased pathway activity scores in the posterior cerebellum of SCA1 mice. We demonstrated unexpectedly large differences in the gene expression between posterior and anterior cerebellar vermis of wild-type mice, indicative of robust intraregional heterogeneity of gene expression in the healthy cerebellum. Additionally, we found that SCA1 disease profoundly reduces intracerebellar heterogeneity of gene expression. Further, using fiber photometry, we found that population level PC calcium activity was altered in the posterior lobules in SCA1 mice during walking. We also identified regional differences in the population level activity of Purkinje cells (PCs) in unrestrained wild-type mice that were diminished in SCA1 mice.

Optimization of systemic AAV9 gene therapy in Niemann-Pick disease type C1 mice

Niemann-Pick disease, type C1 (NPC1) is a rare, fatal neurodegenerative disorder caused by pathological variants in NPC1, which encodes a lysosomal cholesterol transport protein. There are no FDA approved treatments for this disorder. Both systemic and central nervous system delivery of AAV9-hNPC1 have shown significant disease amelioration in NPC1 murine models. To assess the impact of dose and window of therapeutic efficacy in Npc1 m1N mice, we systemically administered three different doses of AAV9-hNPC1 at 4 weeks old and the medium dose at pre-, early, and post-symptomatic timepoints. Higher vector doses and treatment earlier in life were associated with enhanced transduction in the nervous system and resulted in significantly increased lifespan. Similar beneficial effects were noted after gene therapy in Npc1 I1061T mice, a model that recapitulates a common human hypomorphic variant. Our findings help define dose ranges, treatment ages, and efficacy in severe and hypomorphic models of NPC1 deficiency and suggest that earlier delivery of AAV9-hNPC1 in a pre-symptomatic disease state is likely to yield optimal outcomes in individuals with NPC1.

Phenotypical, genotypical and pathological characterization of the moonwalker mouse, a model of ataxia

We performed a comprehensive study of the morphological, functional, and genetic features of moonwalker (MWK) mice, a mouse model of spinocerebellar ataxia caused by a gain of function of the TRPC3 channel. These mice show numerous behavioral symptoms including tremor, altered gait, circling behavior, impaired motor coordination, impaired motor learning and decreased limb strength. Cerebellar pathology is characterized by early and almost complete loss of unipolar brush cells as well as slowly progressive, moderate loss of Purkinje cell (PCs). Structural damage also includes loss of synaptic contacts from parallel fibers, swollen ER structures, and degenerating axons. Interestingly, no obvious correlation was observed between PC loss and severity of the symptoms, as the phenotype stabilizes around 2 months of age, while the cerebellar pathology is progressive. This is probably due to the fact that PC function is severely impaired much earlier than the appearance of PC loss. Indeed, PC firing is already impaired in 3 weeks old mice. An interesting feature of the MWK pathology that still remains to be explained consists in a strong lobule selectivity of the PC loss, which is puzzling considering that TRPC is expressed in every PC. Intriguingly, genetic analysis of MWK cerebella shows, among other alterations, changes in the expression of both apoptosis inducing and resistance factors possibly suggesting that damaged PCs initiate specific cellular pathways that protect them from overt cell loss.

PTRH2 is Necessary for Purkinje Cell Differentiation and Survival and its Loss Recapitulates Progressive Cerebellar Atrophy and Ataxia Seen in IMNEPD Patients

Hom ozygous variants in the peptidyl-tRNA hydrolase 2 gene (PTRH2) cause infantile-onset multisystem neurologic, endocrine, and pancreatic disease. The objective is to delineate the mechanisms underlying the core cerebellar phenotype in this disease. For this, we generated constitutive (Ptrh2LoxPxhCMVCre, Ptrh2-/- mice) and Purkinje cell (PC) specific (Ptrh2LoxPxPcp2Cre, Ptrh2ΔPCmice) Ptrh2 mutant mouse models and investigated the effect of the loss of Ptrh2 on cerebellar development. We show that Ptrh2-/- knockout mice had severe postnatal runting and lethality by postnatal day 14. Ptrh2ΔPC PC specific knockout mice survived until adult age; however, they showed progressive cerebellar atrophy and functional cerebellar deficits with abnormal gait and ataxia. PCs of Ptrh2ΔPC mice had reduced cell size and density, stunted dendrites, and lower levels of ribosomal protein S6, a readout of the mammalian target of rapamycin pathway. By adulthood, there was a marked loss of PCs. Thus, we identify a cell autonomous requirement for PTRH2 in PC maturation and survival. Loss of PTRH2 in PCs leads to downregulation of the mTOR pathway and PC atrophy. This suggests a molecular mechanism underlying the ataxia and cerebellar atrophy seen in patients with PTRH2 mutations leading to infantile-onset multisystem neurologic, endocrine, and pancreatic disease.

The appearance of phagocytic microglia in the postnatal brain of Niemann Pick type C mice is developmentally regulated and underscores shortfalls in fine odor discrimination

The loss of NPC1 or NPC2 function results in cholesterol and sphingolipid dyshomeostasis that impairs developmental trajectories, predisposing the postnatal brain to the appearance of pathological signs, including progressive and stereotyped Purkinje cell loss and microgliosis. Despite increasing evidence reporting the activation of pro-inflammatory microglia as a cardinal event of NPC1 disease progression at symptomatic stages both in patients and preclinical models, how microglia cells respond to altered neurodevelopmental dynamics remains not completely understood. To gain an insight on this issue, we have characterized patterns of microglia activation in the early postnatal cerebellum and young adult olfactory bulb of the hypomorphic Npc1^nmf164 mouse model. Previous evidence has shown that both these areas display a number of anomalies affecting neuron and glial cell proliferation and differentiation, which largely anticipate cellular changes and clinical signs, raising our interest on how microglia interplay to these changes. Even so, to separate the contribution of cues provided by the dysfunctional microenvironment we have also studied microglia isolated from mice of increasing ages and cultured in vitro for 1 week. Our findings show that microglia of both cerebellum and olfactory bulb of Npc1^nmf164 mice adopt an activated phenotype, characterized by increased cell proliferation, enlarged soma size and de-ramified processes, as well as a robust phagocytic activity, in a time- and space-specific manner. Enhanced phagocytosis associates with a profound remodeling of gene expression signatures towards gene products involved in chemotaxis, cell recognition and engulfment, including Cd68 and Trem2. These early changes in microglia morphology and activities are induced by region-specific developmental anomalies that likely anticipate alterations in neuronal connectivity. As a proof of concept, we show that microglia activation within the granule cell layer and glomerular layer of the olfactory bulb of Npc1^nmf164 mice is associated with shortfalls in fine odor discrimination.

A novel genetic variant potentially altering the expression of MANBA in the cerebellum associated with attention deficit hyperactivity disorder in Han Chinese children

OBJECTIVES

To obtain additional insight into the genetic factors of attention deficit hyperactivity disorder (ADHD).

METHODS

First, we performed a transcriptome-wide association study (TWAS) integrating human cerebellum-specific variant-expression/splicing correlations to identify ADHD susceptibility genes. Then, the associations between expression/splicing quantitative trait loci (eQTLs/sQTLs) of the transcriptome-wide significant genes and ADHD were observed in a case-control study of Han Chinese children. Furthermore, dual luciferase reporter gene assays were performed to validate the regulatory function of ADHD risk variants. Additionally, the transcription level of target genes in blood was detected by real-time quantitative polymerase chain reaction (RT-qPCR) assay.

RESULTS

TWAS identified that the genetically regulated expression of MANBA in the cerebellum was significantly associated with ADHD risk. Furthermore, we observed a higher risk of ADHD and more severe clinical symptoms in subjects harbouring heterozygous (TC) or mutant homozygous (TT) genotypes of MANBA rs1054037 than CC carriers. The dual luciferase reporter gene assay revealed that the mutation of rs1054037(C > T) potentially upregulated MANBA expression by eliminating the binding site for hsa-miR-5591-3P. Finally, RT-qPCR showed that MANBA expression in blood samples of patients was significantly higher than that of controls.

CONCLUSIONS

Taken together, these results suggest a role of MANBA in the development of ADHD.

Correlation of age of onset and clinical severity in Niemann-Pick disease type C1 with lysosomal abnormalities and gene expression

Niemann-Pick disease type C1 (NPC1) is a rare, prematurely fatal lysosomal storage disorder which exhibits highly variable severity and disease progression as well as a wide-ranging age of onset, from perinatal stages to adulthood. This heterogeneity has made it difficult to obtain prompt diagnosis and to predict disease course. In addition, small NPC1 patient sample sizes have been a limiting factor in acquiring genome-wide transcriptome data. In this study, primary fibroblasts from an extensive cohort of 41 NPC1 patients were used to validate our previous findings that the lysosomal quantitative probe LysoTracker can be used as a predictor for age of onset and disease severity. We also examined the correlation between these clinical parameters and RNA expression data from primary fibroblasts and identified a set of genes that were significantly associated with lysosomal defects or age of onset, in particular neurological symptom onset. Hierarchical clustering showed that these genes exhibited distinct expression patterns among patient subgroups. This study is the first to collect transcriptomic data on such a large scale in correlation with clinical and cellular phenotypes, providing a rich genomic resource to address NPC1 clinical heterogeneity and discover potential biomarkers, disease modifiers, or therapeutic targets.

Reduction of glutamate neurotoxicity: A novel therapeutic approach for Niemann-Pick disease, type C1

Niemann-Pick disease, type C1 is a progressive, lethal, neurodegenerative disorder due to endolysosomal storage of unesterified cholesterol. Cerebellar ataxia, as a result of progressive loss of cerebellar Purkinje neurons, is a major symptom of Nieman-Pick disease, type C1. Comparing single cell RNAseq data from control (Npc1^+/+) and mutant (Npc1^-/-) mice, we observed significantly decreased expression of Slc1a3 in Npc1^-/- astrocytes. Slc1a3 encodes a glutamate transporter (GLAST, EAAT1) which functions to decrease glutamate concentrations in the post synaptic space after neuronal firing. Glutamate is an excitatory neurotransmitter and elevated extracellular levels of glutamate can be neurotoxic. Impaired EAAT1 function underlies type-6 episodic ataxia, a rare disorder with progressive cerebellar dysfunction, thus suggesting that impaired glutamate uptake in Niemann-Pick disease, type C1 could contribute to disease progression. We now show that decreased expression of Slc1a3 in Npc1^-/- mice has functional consequences that include decreased surface protein expression and decreased glutamate uptake by Npc1^-/- astrocytes. To test whether glutamate neurotoxicity plays a role in Niemann-Pick disease, type C1 progression, we treated NPC1 deficient mice with ceftriaxone and riluzole. Ceftriaxone is a β-lactam antibiotic that is known to upregulate the expression of Slc1a2, an alternative glial glutamate transporter. Although ceftriaxone increased Slc1a2 expression, we did not observe a treatment effect in NPC1 mutant mice. Riluzole is a glutamate receptor antagonist that inhibits postsynaptic glutamate receptor signaling and reduces the release of glutamate. We found that treatment with riluzole increased median survival in Npc1^-/- by 12%. Given that riluzole is an approved drug for the treatment of amyotrophic lateral sclerosis, repurposing of this drug may provide a novel therapeutic approach to decrease disease progression in Niemann-Pick disease type, C1 patients.

Transcriptome of HPβCD-treated Niemann-pick disease type C1 cells highlights GPNMB as a biomarker for therapeutics

The rare, fatal neurodegenerative disorder Niemann-Pick disease type C1 (NPC1) arises from lysosomal accumulation of unesterified cholesterol and glycosphingolipids. These subcellular pathologies lead to phenotypes of hepatosplenomegaly, neurological degeneration and premature death. The timing and severity of NPC1 clinical presentation is extremely heterogeneous. This study analyzed RNA-Seq data from 42 NPC1 patient-derived, primary fibroblast cell lines to determine transcriptional changes induced by treatment with 2-hydroxypropyl-β-cyclodextrin (HPβCD), a compound currently under investigation in clinical trials. A total of 485 HPβCD-responsive genes were identified. Pathway enrichment analysis of these genes showed significant involvement in cholesterol and lipid biosynthesis. Furthermore, immunohistochemistry of the cerebellum as well as measurements of serum from Npc1m1N null mice treated with HPβCD and adeno-associated virus (AAV) gene therapy suggests that one of the identified genes, GPNMB, may serve as a useful biomarker of treatment response in NPC1 disease. Overall, this large NPC1 patient-derived dataset provides a comprehensive foundation for understanding the genomic response to HPβCD treatment.

Region-specific preservation of Purkinje cell morphology and motor behavior in the ATXN1[82Q] mouse model of spinocerebellar ataxia 1

Purkinje cells are the primary processing units of the cerebellar cortex and display molecular heterogeneity that aligns with differences in physiological properties, projection patterns, and susceptibility to disease. In particular, multiple mouse models that feature Purkinje cell degeneration are characterized by incomplete and patterned Purkinje cell degeneration, suggestive of relative sparing of Purkinje cell subpopulations, such as those expressing Aldolase C/zebrinII (AldoC) or residing in the vestibulo‐cerebellum. Here, we investigated a well‐characterized Purkinje cell‐specific mouse model for spinocerebellar ataxia type 1 (SCA1) that expresses human ATXN1 with a polyQ expansion (82Q). Our pathological analysis confirms previous findings that Purkinje cells of the vestibulo‐cerebellum, i.e., the flocculonodular lobes, and crus I are relatively spared from key pathological hallmarks: somatodendritic atrophy, and the appearance of p62/SQSTM1‐positive inclusions. However, immunohistological analysis of transgene expression revealed that spared Purkinje cells do not express mutant ATXN1 protein, indicating the sparing of Purkinje cells can be explained by an absence of transgene expression. Additionally, we found that Purkinje cells in other cerebellar lobules that typically express AldoC, not only display severe pathology but also show loss of AldoC expression. The relatively preserved flocculonodular lobes and crus I showed a substantial fraction of Purkinje cells that expressed the mutant protein and displayed pathology as well as loss of AldoC expression. Despite considerable pathology in these lobules, behavioral analyses demonstrated a relative sparing of related functions, suggestive of sufficient functional cerebellar reserve. Together, the data indicate that mutant ATXN1 affects both AldoC‐positive and AldoC‐negative Purkinje cells and disrupts normal parasagittal AldoC expression in Purkinje cells. Our results show that, in a mouse model otherwise characterized by widespread Purkinje cell degeneration, sparing of specific subpopulations is sufficient to maintain normal performance of specific behaviors within the context of the functional, modular map of the cerebellum.

The rapidly evolving view of lysosomal storage diseases

Lysosomal storage diseases are a group of metabolic disorders caused by deficiencies of several components of lysosomal function. Most commonly affected are lysosomal hydrolases, which are involved in the breakdown and recycling of a variety of complex molecules and cellular structures. The understanding of lysosomal biology has progressively improved over time. Lysosomes are no longer viewed as organelles exclusively involved in catabolic pathways, but rather as highly dynamic elements of the autophagic-lysosomal pathway, involved in multiple cellular functions, including signaling, and able to adapt to environmental stimuli. This refined vision of lysosomes has substantially impacted on our understanding of the pathophysiology of lysosomal disorders. It is now clear that substrate accumulation triggers complex pathogenetic cascades that are responsible for disease pathology, such as aberrant vesicle trafficking, impairment of autophagy, dysregulation of signaling pathways, abnormalities of calcium homeostasis, and mitochondrial dysfunction. Novel technologies, in most cases based on high-throughput approaches, have significantly contributed to the characterization of lysosomal biology or lysosomal dysfunction and have the potential to facilitate diagnostic processes, and to enable the identification of new therapeutic targets.

Coming Back to Physiology: Extra Hepatic Functions of Proprotein Convertase Subtilisin/Kexin Type 9

Neuronal apoptosis regulated convertase-1 (NARC-1), now mostly known as proprotein convertase subtilisin/kexin type 9 (PCSK9), has received a lot of attention due to the fact that it is a key regulator of the low-density lipoprotein (LDL) receptor (LDL-R) and is therefore involved in hepatic LDL clearance. Within a few years, therapies targeting PCSK9 have reached clinical practice and they offer an additional tool to reduce blood cholesterol concentrations. However, PCSK9 is almost ubiquitously expressed in the body but has less well-understood functions and target proteins in extra hepatic tissues. As such, PCSK9 is involved in the regulation of neuronal survival and protein degradation, it affects the expression of the epithelial sodium channel (ENaC) in the kidney, it interacts with white blood cells and with cells of the vascular wall, and it modifies contractile activity of cardiomyocytes, and contributes to the regulation of cholesterol uptake in the intestine. Moreover, under stress conditions, signals from the kidney and heart can affect hepatic expression and thereby the plasma concentration of PCSK9 which then in turn can affect other target organs. Therefore, there is an intense relationship between the local (autocrine) and systemic (endocrine) effects of PCSK9. Although, PCSK9 has been recognized as a ubiquitously expressed modifier of cellular function and signaling molecules, its physiological role in different organs is not well-understood. The current review summarizes these findings.

Altered Capicua expression drives regional Purkinje neuron vulnerability through ion channel gene dysregulation in spinocerebellar ataxia type 1

Selective neuronal vulnerability in neurodegenerative disease is poorly understood. Using the ATXN1[82Q] model of spinocerebellar ataxia type 1 (SCA1), we explored the hypothesis that regional differences in Purkinje neuron degeneration could provide novel insights into selective vulnerability. ATXN1[82Q] Purkinje neurons from the anterior cerebellum were found to degenerate earlier than those from the nodular zone, and this early degeneration was associated with selective dysregulation of ion channel transcripts and altered Purkinje neuron spiking. Efforts to understand the basis for selective dysregulation of channel transcripts revealed modestly increased expression of the ATXN1 co-repressor Capicua (Cic) in anterior cerebellar Purkinje neurons. Importantly, disrupting the association between ATXN1 and Cic rescued the levels of these ion channel transcripts, and lentiviral overexpression of Cic in the nodular zone accelerated both aberrant Purkinje neuron spiking and neurodegeneration. These findings reinforce the central role for Cic in SCA1 cerebellar pathophysiology and suggest that only modest reductions in Cic are needed to have profound therapeutic impact in SCA1.

Toll-like receptor mediated lysozyme expression in Niemann-pick disease, type C1

Niemann-Pick type C1 (NPC1) is a rare neurodegenerative disease. In NPC1 mouse cerebella, the antibacterial enzyme, lysozyme (Lyz2), is significantly increased in multiple cell types. Due to its possible role in toxic fibril deposition, we confirmed Lyz2 overexpression in culture in different control and NPC1 cell types including human NPC1 fibroblasts. Lyz2 expression is induced by Toll-like receptors potentially in response to lipid storage but does not play a functional role in NPC disease pathology.

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.

Evaluation of the Potential Role of Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) in Niemann-Pick Disease, Type C1

Niemann–Pick disease, type C1, is a cholesterol storage disease where unesterified cholesterol accumulates intracellularly. In the cerebellum this causes neurodegeneration of the Purkinje neurons that die in an anterior-to-posterior and time-dependent manner. This results in cerebellar ataxia as one of the major outcomes of the disease. Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a significant role in the regulation of serum cholesterol levels by modulating LDL receptor levels on peripheral tissues. In the central nervous system, PCSK9 may have a similar effect on the closely related VLDL and ApoE2 receptors to regulate brain cholesterol. In addition, regulation of VLDLR and ApoER2 by PCSK9 may contribute to neuronal apoptotic pathways through Reelin, the primary ligand of VLDLR and ApoER2. Defects in reelin signaling results in cerebellar dysfunction leading to ataxia as seen in the Reeler mouse. Our recent findings that Pcsk9 is expressed ~8-fold higher in the anterior lobules of the cerebellum compared to the posterior lobule X, which is resistant to neurodegeneration, prompted us to ask whether PCSK9 could play a role in NPC1 disease progression. We addressed this question genetically, by characterizing NPC1 disease in the presence or absence of PCSK9. Analysis of double mutant Pcsk9^-/-/Npc1^-/- mice by disease severity scoring, motor assessments, lifespan, and cerebellar Purkinje cell staining, showed no obvious difference in NPC1 disease progression with that of Npc1^-/- mice. This suggests that PCSK9 does not play an apparent role in NPC1 disease progression.