No abstract

Despite widespread distribution of LAMP1 and the heterogeneous nature of LAMP1-labeled compartments, LAMP1 is routinely used as a lysosomal marker, and LAMP1-positive organelles are often referred to as lysosomes. In this study, we use immunoelectron microscopy and confocal imaging to provide quantitative analysis of LAMP1 distribution in various autophagic and endolysosomal organelles in neurons. Our study demonstrates that a significant portion of LAMP1-labeled organelles do not contain detectable lysosomal hydrolases including cathepsins D and B and glucocerebrosidase. A bovine serum albumin-gold pulse-chase assay followed by ultrastructural analysis suggests a heterogeneity of degradative capacity in LAMP1-labeled endolysosomal organelles. Gradient fractionation displays differential distribution patterns of LAMP1/2 and cathepsins D/B in neurons. We further reveal that LAMP1 intensity in familial amyotrophic lateral sclerosis-linked motor neurons does not necessarily reflect lysosomal deficits in vivo. Our study suggests that labeling a set of lysosomal hydrolases combined with various endolysosomal markers would be more accurate than simply relying on LAMP1/2 staining to assess neuronal lysosome distribution, trafficking, and functionality under physiological and pathological conditions.

Gaucher's disease (GD), an inherited metabolic disorder caused by mutations in the glucocerebrosidase gene (GBA), is the most common lysosomal storage disease. Heterozygous mutations in GBA are a major risk factor for Parkinson's disease. GD is divided into three clinical subtypes based on the absence (type 1) or presence (types 2 and 3) of neurological signs. Type 1 GD was the first lysosomal storage disease (LSD) for which enzyme therapy became available, and although infusions of recombinant glucocerebrosidase (GCase) ameliorate the systemic effects of GD, the lack of efficacy for the neurological manifestations, along with the considerable expense and inconvenience of enzyme therapy for patients, renders the search for alternative or complementary therapies paramount. Glucosylceramide and glucosylsphingosine accumulation in the brain leads to massive neuronal loss in patients with neuronopathic GD (nGD) and in nGD mouse models. However, the mode of neuronal death is not known. Here, we show that modulating the receptor-interacting protein kinase-3 (Ripk3) pathway markedly improves neurological and systemic disease in a mouse model of GD. Notably, Ripk3 deficiency substantially improved the clinical course of GD mice, with increased survival and motor coordination and salutary effects on cerebral as well as hepatic injury.

SUMMARY Neurons face the challenge of maintaining cellular homeostasis through lysosomal degradation. While enzymatically active degradative lysosomes are enriched in the soma, their axonal trafficking and positioning and impact on axonal physiology remain elusive. Here, we characterized axon-targeted delivery of degradative lysosomes by applying fluorescent probes that selectively label active forms of lysosomal cathepsins D, B, L, and GCase. By time-lapse imaging of cortical neurons in microfluidic devices and standard dishes, we reveal that soma-derived degradative lysosomes rapidly influx into distal axons and target to autophagosomes and Parkinson disease-related α-synuclein cargos for local degradation. Impairing lysosome axonal delivery induces an aberrant accumulation of autophagosomes and α-synuclein cargos in distal axons. Our study demonstrates that the axon is an active compartment for local degradation and reveals fundamental aspects of axonal lysosomal delivery and maintenance. Our work establishes a foundation for investigations into axonal lysosome trafficking and functionality in neurodegenerative diseases.

Gaucher disease has recently received wide attention due to the unexpected discovery that it is a genetic risk factor for Parkinson's disease. Gaucher disease is caused by the defective activity of the lysosomal enzyme, glucocerebrosidase (GCase; GBA1), resulting in intracellular accumulation of the glycosphingolipids, glucosylceramide and psychosine. The rare neuronopathic forms of GD (nGD) are characterized by profound neurological impairment and neuronal cell death. We have previously described the progression of neuropathological changes in a mouse model of nGD. We now examine the relationship between glycosphingolipid accumulation and initiation of pathology at two pre-symptomatic stages of the disease in four different brain areas which display differential degrees of susceptibility to GCase deficiency. Liquid chromatography electrospray ionization tandem mass spectrometry demonstrated glucosylceramide and psychosine accumulation in nGD brains prior to the appearance of neuroinflammation, although only glucosylceramide accumulation correlated with neuroinflammation and neuron loss. Levels of other sphingolipids, including the pro-apoptotic lipid, ceramide, were mostly unaltered. Transmission electron microscopy revealed that glucosylceramide accumulation occurs in neurons, mostly in the form of membrane-delimited pseudo-tubules located near the nucleus. Highly disrupted glucosylceramide-storing cells, which are likely degenerating neurons containing massive inclusions, numerous autophagosomes and unique ultrastructural features, were also observed. Together, our results indicate that a certain level of neuronal glucosylceramide storage is required to trigger neuropathological changes in affected brain areas, while other brain areas containing similar glucosylceramide levels are unaltered, presumably because of intrinsic differences in neuronal properties, or in the neuronal environment, between various brain regions.

Gaucher's disease, the most common lysosomal storage disorder, is caused by the defective activity of glucocerebrosidase, the lysosomal hydrolase that degrades glucosylceramide. The neuronopathic forms of Gaucher's disease are characterized by severe neuronal loss, astrocytosis and microglial proliferation, but the cellular and molecular pathways causing these changes are not known. In the current study, we delineate the role of neuroinflammation in the pathogenesis of neuronopathic Gaucher's disease and show significant changes in levels of inflammatory mediators in the brain of a neuronopathic Gaucher's disease mouse model. Levels of messenger RNA expression of interleukin -1β, tumour necrosis factor-α, tumour necrosis factor-α receptor, macrophage colony-stimulating factor and transforming growth factor-β were elevated by up to ∼30-fold, with the time-course of the increase correlating with the progression of disease severity. The most significant elevation was detected for the chemokines CCL2, CCL3 and CCL5. Blood-brain barrier disruption was also evident in mice with neuronopathic Gaucher's disease. Finally, extensive elevation of nitrotyrosine, a hallmark of peroxynitrite (ONOO(-)) formation, was observed, consistent with oxidative damage caused by macrophage/microglia activation. Together, our results suggest a cytotoxic role for activated microglia in neuronopathic Gaucher's disease. We suggest that once a critical threshold of glucosylceramide storage is reached in neurons, a signalling cascade is triggered that activates microglia, which in turn releases inflammatory cytokines that amplify the inflammatory response, contributing to neuronal death.

Gaucher disease (GD), the most common lysosomal storage disorder, is caused by a deficiency in the lysosomal enzyme glucocerebrosidase (GlcCerase), which results in intracellular accumulation of glucosylceramide (GlcCer). The rare neuronopathic forms of GD are characterized by profound neurological impairment and neuronal cell death, but little is known about the neuropathological changes that underlie these events. We now systematically examine the onset and progression of various neuropathological changes (including microglial activation, astrogliosis and neuron loss) in a mouse model of neuronopathic GD, and document the brain areas that are first affected, which may reflect vulnerability of these areas to GlcCerase deficiency. We also identify neuropathological changes in several brain areas and pathways, such as the substantia nigra reticulata, reticulotegmental nucleus of the pons, cochlear nucleus and the somatosensory system, which could be responsible for some of the neurological manifestations of the human disease. In addition, we establish that microglial activation and astrogliosis are spatially and temporally correlated with selective neuron loss.

BackgroundNeuroinflammation is a key phenomenon in the pathogenesis of many neurodegenerative diseases. Understanding the mechanisms by which brain inflammation is engaged and delineating the key players in the immune response and their contribution to brain pathology is of great importance for the identification of novel therapeutic targets for these devastating diseases. Gaucher disease, the most common lysosomal storage disease, is caused by mutations in the GBA1 gene and is a significant risk factor for Parkinson’s disease; in some forms of Gaucher disease, neuroinflammation is observed.MethodsAn unbiased gene profile analysis was performed on a severely affected brain area of a neurological form of a Gaucher disease mouse at a pre-symptomatic stage; the mouse used for this study, the Gbaflox/flox; nestin-Cre mouse, was engineered such that GBA1 deficiency is restricted to cells of neuronal lineage, i.e., neurons and macroglia.ResultsThe 10 most up-regulated genes in the ventral posteromedial/posterolateral region of the thalamus were inflammatory genes, with the gene expression signature significantly enriched in interferon signaling genes. Interferon β levels were elevated in neurons, and interferon-stimulated genes were elevated mainly in microglia. Interferon signaling pathways were elevated to a small extent in the brain of another lysosomal storage disease mouse model, Krabbe disease, but not in Niemann-Pick C or Sandhoff mouse brain. Ablation of the type I interferon receptor attenuated neuroinflammation but had no effect on GD mouse viability.ConclusionsOur results imply that the type I interferon response is involved in the development of nGD pathology, and possibly in other lysosomal storage diseases in which simple glycosphingolipids accumulate, and support the notion that interferon signaling pathways play a vital role in the sterile inflammation that often occurs during chronic neurodegenerative diseases in which neuroinflammation is present.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-016-0570-2) contains supplementary material, which is available to authorized users.