Gaucher disease: a lysosomal neurodegenerative disorder

Gaucher disease is a multisystemic disorder that affects men and woman in equal numbers and occurs in all ethnic groups at any age with racial variations and an estimated worldwide incidence of 1/75,000. It is caused by a genetic deficient activity of the lysosomal enzyme glucocerebrosidase due to mutations in the β-glucocerebrosidase gene, and resulting in lack of glucocerebroside degradation. The subsequent accumulation of glucocerebroside in lysosomes of tissue macrophages primarily in the liver, bone marrow and spleen, causes damage in haematological, skeletal and nervous systems. The clinical manifestations show a high degree of variability with symptoms that varies according to organs involved. In many cases, these disorders do not correlate with mutations in the β-glucocerebrosidase gene. Although several mutations have been identified as responsible for the deficient activity of glucocerebrosidase, mechanisms by which this enzymatic defect leads to Gaucher disease remain poorly understood. Recent reports indicate the implication of complex mechanisms, including enzyme deficiency, substrate accumulation, unfolded protein response, and macrophage activation. Further elucidating these mechanisms will advance understanding of Gaucher disease and related disorders.

Dietary modifications in patients receiving miglustat

Weight loss and gastrointestinal disturbances are often seen during miglustat therapy for lysosomal storage diseases. A retrospective analysis of data from a mixed group of patients treated with miglustat at two UK centres was performed to evaluate the effect of two different dietary interventions on body weight and gastrointestinal tolerability during the initial 6 months of miglustat therapy. Neurological outcomes in these patients are not discussed herein. Data were analysed from a total of 29 patients with varied neurolipidoses (21 children/adolescents; 8 adults). Negative mean changes in body weight were seen in children/adolescents on an unmodified diet (-8.1%), and in adults (-4.1%) and children/adolescents (-5.2%) on a low-lactose diet. Patients on the low-disaccharide diet showed a positive mean change in body weight (+2.0%), although there was high variability in this group. Non-parametric sub-analysis of median body-weight change in children/adolescents also showed high variability both within and between diet groups, with no statistically significant difference between the effects of different diets on body weight (p = 0.062). The low-lactose diet reduced gastrointestinal disturbances; single small doses of loperamide were required in some patients. Patients on the low-disaccharide diet showed the lowest frequency of gastrointestinal effects. In conclusion, simple dietary modifications allowed the maintenance of body-weight gain in line with normal growth potential during miglustat therapy in young patients with lysosomal storage diseases, and reduced gastrointestinal disturbances.

Molecular Pathologies of and Enzyme Replacement Therapies for Lysosomal Diseases

Lysosomal diseases comprise a group of inherited disorders resulting from defects of lysosomal enzymes and their cofactors, and in many of them the nervous system is affected. Recently, enzyme replacement therapy with recombinant lysosomal enzymes has been clinically available for several lysosomal diseases. Such enzyme replacement therapies can improve non-neurological disorders but is not effective for neurological ones. In this review, we discuss the molecular pathologies of lysosomal diseases from the protein structural aspect, current enzyme replacement therapies, and attempts to develop enzyme replacement therapies effective for lysosomal diseases associated with neurological disorders, i.e., production of enzymes, brain-specific delivery and incorporation of lysosomal enzymes into cells.

[Tripeptidyl-peptidase I--distribution, biogenesis, and mechanisms of activation]

Tripeptidyl-peptidase I (TPPI) is an acidic lysosomal peptidase that removes tripeptides from an unmodified N-terminus of small proteins and polypeptides. In humans, TPP I constitutes an integral part of the lysosomal proteolytic apparatus, which, includes numerous hydrolytic enzymes, mostly cysteine proteases (cathepsin B, C, H, K, L, and others), but also serine (cathepsin A) and aspartic (cathepsin D) proteases. The combination of endo- and exopeptidase activities of these enzymes allows for efficient digestion of the diverse proteins transported to the lysosomes, releasing free amino acids and dipeptides that are transported back to the cytoplasm and reused according to the metabolic needs of the cell. The role of TPP I in normal lysosome functioning is underscored by the genetic association of the enzyme with one form of a group of the developmental neurodegenerative disorders of childhood--the neuronal ceroid lipofuscinoses (NCLs). The scope of this article is to review the most recent data, mostly from author's laboratory, on the biology and pathology of TPP I. NCLs are also shortly reviewed with the special emphasis on CLN2 form resulting from mutations in TPP I gene.

Disruption of PPT2 in mice causes an unusual lysosomal storage disorder with neurovisceral features

The palmitoyl protein thioesterase-2 (PPT2) gene encodes a lysosomal thioesterase homologous to PPT1, which is the enzyme defective in the human disorder called infantile neuronal ceroid lipofuscinosis. In this article, we report that PPT2 deficiency in mice causes an unusual form of neuronal ceroid lipofuscinosis with striking visceral manifestations. All PPT2-deficient mice displayed a neurodegenerative phenotype with spasticity and ataxia by 15 mo. The bone marrow was infiltrated by brightly autofluorescent macrophages and multinucleated giant cells, but interestingly, the macrophages did not have the typical appearance of foam cells commonly associated with other lysosomal storage diseases. Marked splenomegaly caused by extramedullary hematopoiesis was observed. The pancreas was grossly orange to brown as a result of massive storage of lipofuscin pigments in the exocrine (but not islet) cells. Electron microscopy showed that the storage material consisted of multilamellar membrane profiles ("zebra bodies"). In summary, PPT2 deficiency in mice manifests as a neurodegenerative disorder with visceral features. Although PPT2 deficiency has not been described in humans, manifestations would be predicted to include neurodegeneration with bone marrow histiocytosis, visceromegaly, brown pancreas, and linkage to chromosome 6p21.3 in affected families.

Distribution of tripeptidyl peptidase I in human tissues under normal and pathological conditions

Tripeptidyl peptidase I (TPP I) is a lysosomal exopeptidase that cleaves tripeptides from the free N-termini of oligopeptides. Mutations in this enzyme are associated with the classic late-infantile form of neuronal ceroid lipofuscinosis (CLN2), an autosomal recessive disorder leading to severe brain damage. To gain more insight into CLN2 pathogenesis and the role of TPP I in human tissues in general, we analyzed the temporal and spatial distribution of TPP I in the brain and its localization in internal organs under normal and pathological conditions. We report that TPP I immunoreactivity appears in neurons late in gestation and increases gradually in the postnatal period, matching significantly the final differentiation and maturation of neural tissue. Endothelial cells, choroid plexus, microglial cells, and ependyma showed TPP I immunostaining distinctly earlier than neurons. Acquisition of the adult pattern of TPP I distribution in the brain at around the age of 2 years correlates with the onset of clinical signs in CLN2 subjects. In adults, TPP I was found in all types of cells in the brain and internal organs we studied, although the intensity of TPP I labeling varied among several types of cells and showed a noticeable predilection for cells and/or organs associated with peptide hormone and neuropeptide production. In addition, TPP I immunoreactivity was increased in aging brain, neurodegenerative and lysosomal storage disorders, and some differentiated neoplasms and was reduced in ischemic/anoxic areas and undifferentiated tumors. These findings suggest that TPP I is involved in general protein turnover and that its expression may be controlled by various regulatory mechanisms, which highlights the importance of this enzyme for normal function of cells and organs in humans.

Cathepsin D deficiency induces lysosomal storage with ceroid lipofuscin in mouse CNS neurons

Cathepsin D-deficient (CD-/-) mice have been shown to manifest seizures and become blind near the terminal stage [approximately postnatal day (P) 26]. We therefore examined the morphological, immunocytochemical, and biochemical features of CNS tissues of these mice. By electron microscopy, autophagosome/autolysosome-like bodies containing part of the cytoplasm, granular osmiophilic deposits, and fingerprint profiles were demonstrated in the neuronal perikarya of CD-/- mouse brains after P20. Autophagosomes and granular osmiophilic deposits were detected in neurons at P0 but were few in number, whereas they increased in the neuronal perikarya within days after birth. Some large-sized neurons having autophagosome/autolysosome-like bodies in the perikarya appeared in the CNS tissues, especially in the thalamic region and the cerebral cortex, at P17. These lysosomal bodies occupied the perikarya of almost all neurons in CD-/- mouse brains obtained from P23 until the terminal stage. Because these neurons exhibited autofluorescence, it was considered that ceroid lipofuscin may accumulate in lysosomal structures of CD-/- neurons. Subunit c of mitochondrial ATP synthase was found to accumulate in the lysosomes of neurons, although the activity of tripeptidyl peptidase-I significantly increased in the brain. Moreover, neurons near the terminal stage were often shrunken and possessed irregular nuclei through which small dense chromatin masses were scattered. These results suggest that the CNS neurons in CD-/- mice show a new form of lysosomal accumulation disease with a phenotype resembling neuronal ceroid lipofuscinosis.