Stimulation of mGluR1/5 Improves Defective Internalization of AMPA Receptors in NPC1 Mutant Mouse

Impact of Organelle Transport Deficits on Mitophagy and Autophagy in Niemann-Pick Disease Type C

Defective mitochondria are pathophysiological features of a number of neurodegenerative diseases. Here, we investigated mitochondrial dysfunction in the context of the rare lysosomal storage diseases Niemann–Pick disease type C1 and type C2 (NP-C1 and NP-C2). Mutations in either the NPC1 or NPC2 gene lead to cholesterol accumulation in late endosomes and lysosomes, resulting in impaired cholesterol homeostasis. The extent to which this may lead to mitochondrial dysfunction has been poorly studied so far. Therefore, we investigated the morphology, function, and transport of mitochondria, as well as their degradation via mitophagy, in a disease-associated human neural cell model of NP-C. By performing live cell imaging, we observed markedly reduced mitochondrial transport, although morphology and function were not appreciably altered. However, we observed a defective mitophagy induction shown by a reduced capability to elevate parkin expression and engulf mitochondria in autophagosomes after treatment with carbonyl cyanide 3-chlorophenylhydrazone (CCCP). This was accompanied by defects in autophagy induction, exhibited by a hampered p62 expression and progression, shown by increased LC3BII levels and a defective fusion of autophagosomes and lysosomes. The latter might have been additionally influenced by the observed reduced lysosomal transport. Hence, we hypothesized that a reduced recycling of mitochondria contributes to the pathophysiology of NP-C.

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.

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.

Deregulation of signalling in genetic conditions affecting the lysosomal metabolism of cholesterol and galactosyl-sphingolipids

The role of lipids in neuroglial function is gaining momentum in part due to a better understanding of how many lipid species contribute to key cellular signalling pathways at the membrane level. The description of lipid rafts as membrane domains composed by defined classes of lipids such as cholesterol and sphingolipids has greatly helped in our understanding of how cellular signalling can be regulated and compartmentalized in neurons and glial cells. Genetic conditions affecting the metabolism of these lipids greatly impact on how some of these signalling pathways work, providing a context to understand the biological function of the lipid. Expectedly, abnormal metabolism of several lipids such as cholesterol and galactosyl-sphingolipids observed in several metabolic conditions involving lysosomal dysfunction are often accompanied by neuronal and myelin dysfunction. This review will discuss the role of lysosomal biology in the context of deficiencies in the metabolism of cholesterol and galactosyl-sphingolipids and their impact on neural function in three genetic disorders: Niemann-Pick type C, Metachromatic leukodystrophy and Krabbe’s disease.

Ischemia-Triggered Glutamate Excitotoxicity From the Perspective of Glial Cells

A plethora of neurological disorders shares a final common deadly pathway known as excitotoxicity. Among these disorders, ischemic injury is a prominent cause of death and disability worldwide. Brain ischemia stems from cardiac arrest or stroke, both responsible for insufficient blood supply to the brain parenchyma. Glucose and oxygen deficiency disrupts oxidative phosphorylation, which results in energy depletion and ionic imbalance, followed by cell membrane depolarization, calcium (Ca²⁺) overload, and extracellular accumulation of excitatory amino acid glutamate. If tight physiological regulation fails to clear the surplus of this neurotransmitter, subsequent prolonged activation of glutamate receptors forms a vicious circle between elevated concentrations of intracellular Ca²⁺ ions and aberrant glutamate release, aggravating the effect of this ischemic pathway. The activation of downstream Ca²⁺-dependent enzymes has a catastrophic impact on nervous tissue leading to cell death, accompanied by the formation of free radicals, edema, and inflammation. After decades of “neuron-centric” approaches, recent research has also finally shed some light on the role of glial cells in neurological diseases. It is becoming more and more evident that neurons and glia depend on each other. Neuronal cells, astrocytes, microglia, NG2 glia, and oligodendrocytes all have their roles in what is known as glutamate excitotoxicity. However, who is the main contributor to the ischemic pathway, and who is the unsuspecting victim? In this review article, we summarize the so-far-revealed roles of cells in the central nervous system, with particular attention to glial cells in ischemia-induced glutamate excitotoxicity, its origins, and consequences.

Alteration of GABAergic Input Precedes Neurodegeneration of Cerebellar Purkinje Cells of NPC1-Deficient Mice

Niemann-Pick Disease Type C1 (NPC1) is a rare hereditary neurodegenerative disease belonging to the family of lysosomal storage disorders. NPC1-patients suffer from, amongst other symptoms, ataxia, based on the dysfunction and loss of cerebellar Purkinje cells. Alterations in synaptic transmission are believed to contribute to a pathological mechanism leading to the progressive loss of Purkinje cells observed in NPC1-deficient mice. With regard to inhibitory synaptic transmission, alterations of GABAergic synapses are described but functional data are missing. For this reason, we have examined here the inhibitory GABAergic synaptic transmission of Purkinje cells of NPC1-deficient mice (NPC1^−/−). Patch clamp recordings of inhibitory post-synaptic currents (IPSCs) of Purkinje cells revealed an increased frequency of GABAergic IPSCs in NPC1^−/− mice. In addition, Purkinje cells of NPC1^−/− mice were less amenable for modulation of synaptic transmission via the activation of excitatory NMDA-receptors (NMDA-Rs). Western blot testing disclosed a reduced protein level of phosphorylated alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPA-Rs) subunit GluA2 in the cerebella of NPC1^−/− mice, indicating a disturbance in the internalization of GluA2-containing AMPA-Rs. Since this is triggered by the activation of NMDA-Rs, we conclude that a disturbance in the synaptic turnover of AMPA-Rs underlies the defective inhibitory GABAergic synaptic transmission. While these alterations precede obvious signs of neurodegeneration of Purkinje cells, we propose a contribution of synaptic malfunction to the initiation of the loss of Purkinje cells in NPC1. Thus, a prevention of the disturbance of synaptic transmission in early stages of the disease might display a target with which to avert progressive neurodegeneration in NPC1.