Loss of CLN7 results in depletion of soluble lysosomal proteins and impaired mTOR reactivation.

Defects in the MFSD8 gene encoding the lysosomal membrane protein CLN7 lead to CLN7 disease, a neurodegenerative lysosomal storage disorder belonging to the group of neuronal ceroid lipofuscinoses. Here, we have performed a SILAC-based quantitative analysis of the lysosomal proteome using Cln7-deficient mouse embryonic fibroblasts (MEFs) from a Cln7 knockout (ko) mouse model. From 3335 different proteins identified, we detected 56 soluble lysosomal proteins and 29 highly abundant lysosomal membrane proteins. Quantification revealed that the amounts of 12 different soluble lysosomal proteins were significantly reduced in Cln7 ko MEFs compared with wild-type controls. One of the most significantly depleted lysosomal proteins was Cln5 protein that underlies another distinct neuronal ceroid lipofuscinosis disorder. Expression analyses showed that the mRNA expression, biosynthesis, intracellular sorting and proteolytic processing of Cln5 were not affected, whereas the depletion of mature Cln5 protein was due to increased proteolytic degradation by cysteine proteases in Cln7 ko lysosomes. Considering the similar phenotypes of CLN5 and CLN7 patients, our data suggest that depletion of CLN5 may play an important part in the pathogenesis of CLN7 disease. In addition, we found a defect in the ability of Cln7 ko MEFs to adapt to starvation conditions as shown by impaired mammalian target of rapamycin complex 1 reactivation, reduced autolysosome tubulation and increased perinuclear accumulation of autolysosomes compared with controls. In summary, depletion of multiple soluble lysosomal proteins suggest a critical role of CLN7 for lysosomal function, which may contribute to the pathogenesis and progression of CLN7 disease.

Endo-lysosomal and autophagic dysfunction: a driving factor in Alzheimer's disease?

Alzheimer's disease (AD) is the most common cause of dementia, and its prevalence will increase significantly in the coming decades. Although important progress has been made, fundamental pathogenic mechanisms as well as most hereditary contributions to the sporadic form of the disease remain unknown. In this review, we examine the now substantial links between AD pathogenesis and lysosomal biology. The lysosome hydrolyses and processes cargo delivered by multiple pathways, including endocytosis and autophagy. The endo-lysosomal and autophagic networks are central to clearance of cellular macromolecules, which is important given there is a deficit in clearance of amyloid-ß in AD. Numerous studies show prominent lysosomal dysfunction in AD, including perturbed trafficking of lysosomal enzymes and accumulation of the same substrates that accumulate in lysosomal storage disorders. Examination of the brain in lysosomal storage disorders shows the accumulation of amyloid precursor protein metabolites, which further links lysosomal dysfunction with AD. This and other evidence leads us to hypothesise that genetic variation in lysosomal genes modifies the disease course of sporadic AD.

Lessons learnt from animal models: pathophysiology of neuropathic lysosomal storage disorders.

Approximately 50 inborn errors of metabolism known as lysosomal storage disorders have been discovered to date, most of which are due to a single mutation in a gene encoding a soluble lysosomal enzyme. Consequently, inadequate enzyme activity results in the accumulation of substrates for that enzyme, invariably accompanied by a wide variety of secondary pathological changes. Many of these conditions remain untreatable, and therefore, research into pathogenic processes and potential treatment strategies is intense. A key tool for researchers in this area is the availability of clinically relevant animal models in which to study disease manifestation and evaluate therapeutic outcomes. Large numbers of both naturally occurring and genetically modified animal models of neurodegenerative lysosomal storage disorders are in existence, with spontaneous models occurring in both large domestic (e.g., cat, dog, sheep) and small (e.g., mouse) animal species. Many have undergone rigorous phenotypic characterization and are now providing us with insights into neurological disease processes. The purpose of this review is to highlight some of the major lessons learnt from these studies.

Clinical aspects of neuropathic lysosomal storage disorders.

The purpose of this review is to describe neurological phenotypes associated with lysosomal storage diseases (LSDs), focusing on features arising from primary neuronal involvement. Clinical presentation, progression and genetic data, are discussed in detail in Part 2, the electronic material. Main features are summarized in Part 1. Insights gained from several observational studies are discussed. Prospective studies of the natural history of most neuronopathic LSDs have been hampered by the rarity of these conditions and the short survival of affected patients. Increasingly, longitudinal observations relating to neurological manifestations are being reported. Better clinical studies are necessary, including repeated measurements of disease progression to facilitate the development of sensitive scoring systems and appropriate counseling of affected individuals and their families. Ideally, clinical studies should involve a large cohort. As treatment becomes available, knowledge of disease expression and factors that influence the phenotype may enable critical assessment of therapeutic outcomes. It is hoped that increased familiarity with the clinical expression of individual LSDs will allow early diagnosis, so families at risk are given options to consider during future pregnancies. Early diagnosis also permits the introduction of timely intervention, to favoring improved outcome in cases that are potentially treatable.

Pathophysiology of neuropathic lysosomal storage disorders.

Although neurodegenerative diseases are most prevalent in the elderly, in rare cases, they can also affect children. Lysosomal storage diseases (LSDs) are a group of inherited metabolic neurodegenerative disorders due to deficiency of a specific protein integral to lysosomal function, such as enzymes or lysosomal components, or to errors in enzyme trafficking/targeting and defective function of nonenzymatic lysosomal proteins, all preventing the complete degradation and recycling of macromolecules. This primary metabolic event determines a cascade of secondary events, inducing LSD's pathology. The accumulation of intermediate degradation affects the function of lysosomes and other cellular organelles. Accumulation begins in infancy and progressively worsens, often affecting several organs, including the central nervous system (CNS). Affected neurons may die through apoptosis or necrosis, although neuronal loss usually does not occur before advanced stages of the disease. CNS pathology causes mental retardation, progressive neurodegeneration, and premature death. Many of these features are also found in adult neurodegenerative disorders, such as Alzheimer's, Parkinson's, and Huntington's diseases. However, the nature of the secondary events and their exact contribution to mental retardation and dementia remains largely unknown. Recently, lysosomal involvement in the pathogenesis of these disorders has been described. Improved knowledge of secondary events may have impact on diagnosis, staging, and follow-up of affected children. Importantly, new insights may provide indications about possible disease reversal upon treatment. A discussion about the CNS pathophysiology involvement in LSDs is the aim of this review. The lysosomal involvement in adult neurodegenerative diseases will also be briefly described.

A double TRPtych: six views of transient receptor potential channels in disease and health.

At the 2008 Annual Meeting of the Society for Neuroscience, a Mini-Symposium entitled "Contributions to TRP Channels to Neurological Disease" included talks from six heads of newly established laboratories, each with a unique research focus, model system, and set of experimental tools. Some of the questions addressed in these talks include the following. What is the role of transient receptor potential (TRP) channels in pain perception? How do normally functioning TRP channels contribute to cell death pathways? What are the characteristics of TRPpathies, disease states that result from overactive or underactive TRP channels? How are TRP channels regulated by signal transduction cascades? This review summarizes recent results from those laboratories and provides six perspectives on the subject of TRP channels and disease.

Monitoring disease evolution and treatment response in lysosomal disorders by the peripheral benzodiazepine receptor ligand PK11195.

Microglia activation and neuroinflammation play a pivotal role in the pathogenesis of lysosomal storage disorders (LSD) affecting the central nervous system (CNS), which are amenable to treatment by hematopoietic stem cell transplantation (HSCT). HSCT efficacy relies on replacing the intra- and extra-vascular hematopoietic cell compartments, including CNS microglia, with a cell population expressing the functional enzyme. Non-invasive and quantitative assessment of microglia activation and of its reduction upon HSCT might allow for evaluation of disease evolution and response to treatment in LSD. We here demonstrate that microglia activation can be quantified ex vivo and in vivo by PET using the peripheral benzodiazepine receptor ligand PK11195 in two models of LSD. Furthermore, we show a differential PBR binding following microglia replacement by donor cells in mice undergoing HSCT. Our data indicates that PBR ligands constitute valuable tools for monitoring the evolution and the response to treatment of LSD with CNS involvement, and enable us to evaluate whether the turnover between endogenous and donor microglia following HSCT could be adequate enough to delay disease progression.

Spinsters, synaptic defects, and amaurotic idiocy.

In this issue of Neuron, Sweeney and Davis present a beautiful characterization of Drosophila mutants in a gene named spinster. The results indicate a function of the endocytic pathway in regulating transforming growth factor-beta (TGF-beta) signaling at the Drosophila motor synapse. This study provides important new information at an intersection of several disciplines, including membrane traffic, lipid organization, synaptic signaling, and neurodegenerative lysosomal storage disease.

Central and peripheral nervous system dysfunction in the clinical variation of Salla disease.

OBJECTIVE: To evaluate the degree of possible peripheral nervous system (PNS) involvement in addition to CNS manifestations in Salla disease, a free sialic acid storage disorder leading to severe mental retardation with a wide clinical variation. BACKGROUND: Salla disease is a lysosomal storage disorder that affects the white matter of the CNS. MRI findings and recent 1H MRS study results provide evidence for delayed central myelination, but there is no previous evidence for PNS involvement in this disease. The gene coding for a presumptive sialic acid transport protein has recently been identified, and the first disease-causing mutations have been characterized. METHODS: Nerve conduction studies; evoked potentials to visual (VEP), brainstem auditory (BAEP), and somatosensory stimuli (SEP); and EEG were carried out on 22 patients (age range 2 months to 57 years) with biochemically and genetically confirmed Salla disease. Brain MRI were available on 14 patients. RESULTS: Nerve conduction studies revealed abnormalities in nearly half of the patients (10/21). The four severely disabled patients and the oldest patient had greatly reduced nerve conduction velocities and prolonged distal latencies compatible with demyelinating polyneuropathy. In addition, SEP was abnormal in the majority of the patients, but VEP and BAEP in only a few cases. PNS involvement was clearly associated with both the phenotypic severity and MRI findings. CONCLUSIONS: The results indicate that dysmyelination in Salla disease occurs not only in the CNS but also in the peripheral nervous system, contributing to the phenotypic variation, which can now be correlated with the molecular basis of the disease.

Startle epilepsy complicating aspartylglucosaminuria.

A 21-year-old right-handed man with definite diagnosis of aspartylglucosaminuria (AGU) presented with a 5-year history of progressive severe gait disturbance with frequent falls and generalized epileptic seizures triggered by unexpected stimuli. At one time, he was confined to a wheelchair because of the frequent falls. Electromyogram recording showed a large, excessive and not habituating motor startle response, with the classical and stereotyped order of muscle recruitment. During video-polygraphic recording, we recorded a reflex generalized tonic seizure triggered by a loud, unexpected acoustic stimulus. Brain magnetic resonance (MR) revealed no structural abnormality. A diagnosis of abnormal startle and startle epilepsy (SE) was made. The addition of clonazepam to valproate and phenobarbital led to a dramatic improvement in his abnormal startle and SE, and the patient was able to walk alone unaided. This report illustrates, for the first time, that abnormal startle and SE may occur in AGU and complicate its clinical picture. Recognition of this entity in AGU is important, as progressive gait disorder with frequent falls could be easily misinterpreted as an additional irreversible manifestation of the ongoing neurological deterioration characteristic of AGU.

Neuroimaging of lipid storage disorders.

Lipid storage diseases, also known as the lipidoses, are a group of inherited metabolic disorders in which there is lipid accumulation in various cell types, including the central nervous system, because of the deficiency of a variety of enzymes. Over time, excessive storage can cause permanent cellular and tissue damage. The brain is particularly sensitive to lipid storage as the contents of the central nervous system must occupy uniform volume, and any increases in fluids or deposits will lead to pressure changes and interference with normal neurological function. In addition to primary lipid storage diseases, lysosomal storage diseases include the mucolipidoses (in which excessive amounts of lipids and carbohydrates are stored in the cells and tissues) and the mucopolysaccharidoses (in which abnormal glycosylated proteins cannot be broken down because of enzyme deficiency). Neurological dysfunction can be a manifestation of these conditions due to substrate deposition as well. This review will explore the modalities of neuroimaging that may have particular relevance to the study of the lipid storage disorder and their impact on elucidating aspects of brain function. First, the techniques will be reviewed. Next, the neuropathology of a few selected lipid storage disorders will be reviewed and the use of neuroimaging to define disease characteristics discussed in further detail. Examples of studies using these techniques will be discussed in the text.

A block of autophagy in lysosomal storage disorders.

Most lysosomal storage disorders (LSDs) are caused by deficiencies of lysosomal hydrolases. While LSDs were among the first inherited diseases for which the underlying biochemical defects were identified, the mechanisms from enzyme deficiency to cell death are poorly understood. Here we show that lysosomal storage impairs autophagic delivery of bulk cytosolic contents to lysosomes. By studying the mouse models of two LSDs associated with severe neurodegeneration, multiple sulfatase deficiency (MSD) and mucopolysaccharidosis type IIIA (MPSIIIA), we observed an accumulation of autophagosomes resulting from defective autophagosome-lysosome fusion. An impairment of the autophagic pathway was demonstrated by the inefficient degradation of exogenous aggregate-prone proteins (i.e. expanded huntingtin and mutated alpha-synuclein) in cells from LSD mice. This impairment resulted in massive accumulation of polyubiquitinated proteins and of dysfunctional mitochondria which are the putative mediators of cell death. These data identify LSDs as 'autophagy disorders' and suggest the presence of common mechanisms in the pathogenesis of these and other neurodegenerative diseases.

Neuronal and glial accumulation of alpha- and beta-synucleins in human lipidoses.

A number of the lysosomal storage diseases that have now been characterized are associated with intra-lysosomal accumulation of lipids, caused by defective lysosomal enzymes. We have previously reported neuronal accumulation of both alpha- and beta-synucleins in brain tissue of a GM2 gangliosidosis mouse model. Although alpha-synuclein has been implicated in several neurodegenerative disorders including Parkinson's disease, dementia with Lewy bodies and multiple system atrophy, its functions remain largely unclear. In our present study, we have examined a cohort of human lipidosis cases, including Sandhoff disease, Tay-Sachs disease, metachromatic leukodystrophy, beta-galactosialidosis and adrenoleukodystrophy, for the expression of alpha- and beta-synucleins and the associated lipid storage levels. The accumulation of alpha-synuclein was found in brain tissue in not only cases of lysosomal storage diseases, but also in instances of adrenoleukodystrophy, which is a peroxisomal disease. alpha-synuclein was detected in both neurons and glial cells of patients with these two disorders, although its distribution was found to be disease-dependent. In addition, alpha-synuclein-positive neurons were also found to be NeuN-positive, whereas NeuN-negative neurons did not show any accumulation of this protein. By comparison, the accumulation of beta-synuclein was detectable only in the pons of Sandhoff disease cases. This differential accumulation of alpha- and beta-synucleins in human lipidoses may be related to functional differences between these two proteins. In addition, the accumulation of alpha-synuclein may also be a condition that is common to lysosomal storage diseases and adrenoleukodystrophies that show an enhanced expression of this protein upon the elevation of stored lipids.

Alpha-mannosidosis in the guinea pig: a new animal model for lysosomal storage disorders.

Alpha-mannosidosis is a lysosomal storage disorder resulting from deficient activity of lysosomal alpha-mannosidase. It has been described previously in humans, cattle, and cats, and is characterized in all of these species principally by neuronal storage leading to progressive mental deterioration. Two guinea pigs with stunted growth, progressive mental dullness, behavioral abnormalities, and abnormal posture and gait, showed a deficiency of acidic alpha-mannosidase activity in leukocytes, plasma, fibroblasts, and whole liver extracts. Fractionation of liver demonstrated a deficiency of lysosomal (acidic) alpha-mannosidase activity. Thin layer chromatography of urine and tissue extracts confirmed the diagnosis by demonstrating a pattern of excreted and stored oligosaccharides almost identical to that of urine from a human alpha-mannosidosis patient. Widespread neuronal vacuolation was observed throughout the CNS, including the cerebral cortex, hippocampus, thalamus, cerebellum, midbrain, pons, medulla, and the dorsal and ventral horns of the spinal cord. Lysosomal vacuolation also occurred in many other visceral tissues and was particularly severe in pancreas, thyroid, epididymis, and peripheral ganglion. Axonal spheroids were observed in some brain regions, but gliosis and demyelination were not observed. Ultrastructurally, most vacuoles in both the CNS and visceral tissues were lucent or contained fine fibrillar or flocculent material. Rare large neurons in the cerebral cortex contained fine membranous structures. Skeletal abnormalities were very mild. Alpha-mannosidosis in the guinea pig closely resembles the human disease and will provide a convenient model for investigation of new therapeutic strategies for neuronal storage diseases, such as enzyme replacement and gene replacement therapies.

The neuropathogenic contributions of lysosomal dysfunction.

Multiple lines of evidence implicate lysosomes in a variety of pathogenic events that produce neurodegeneration. Genetic mutations that cause specific enzyme deficiencies account for more than 40 lysosomal storage disorders. These mostly pre-adult diseases are associated with abnormal brain development and mental retardation. Such disorders are characterized by intracellular deposition and protein aggregation, events also found in age-related neurodegenerative diseases including (i) Alzheimer's disease and related tauopathies (ii) Lewy body disorders and synucleinopathies such as Parkinson's disease, and (iii) Huntington's disease and other polyglutamine expansion disorders. Of particular interest for this review is evidence that alterations to the lysosomal system contribute to protein deposits associated with different types of age-related neurodegeneration. Lysosomes are in fact highly susceptible to free radical oxidative stress in the aging brain, leading to the gradual loss of their processing capacity over the lifespan of an individual. Several studies point to this lysosomal disturbance as being involved in amyloidogenic processing, formation of paired helical filaments, and the aggregation of alpha-synuclein and mutant huntingtin proteins. Most notably, experimentally induced lysosomal dysfunction, both in vitro and in vivo, recapitulates important pathological features of age-related diseases including the link between protein deposition and synaptic loss.