A mutation in the ovine cathepsin D gene causes a congenital lysosomal storage disease with profound neurodegeneration

A new large animal model of CLN5 neuronal ceroid lipofuscinosis in Borderdale sheep is caused by a nucleotide substitution at a consensus splice site (c.571+1G>A) leading to excision of exon 3

Batten disease (neuronal ceroid lipofuscinoses, NCLs) are a group of inherited childhood diseases that result in severe brain atrophy, blindness and seizures, leading to premature death. To date, eight different genes have been identified, each associated with a different form. Linkage analysis indicated a CLN5 form in a colony of affected New Zealand Borderdale sheep. Sequencing studies established the disease-causing mutation to be a substitution at a consensus splice site (c.571+1G>A), leading to the excision of exon 3 and a truncated putative protein. A molecular diagnostic test has been developed based on the excision of exon 3. Sequence alignments support the gene product being a soluble lysosomal protein. Western blotting of isolated storage bodies indicates the specific storage of subunit c of mitochondrial ATP synthase. This flock is being expanded as a large animal model for mechanistic studies and trial therapies.

Haematopoietic development and immunological function in the absence of cathepsin D

BACKGROUND

Cathepsin D is a well-characterized aspartic protease expressed ubiquitously in lysosomes. Cathepsin D deficiency is associated with a spectrum of pathologies leading ultimately to death. Cathepsin D is expressed at high levels in many cells of the immune system, but its role in immune function is not well understood. This study examines the reconstitution and function of the immune system in the absence of cathepsin D, using bone marrow radiation chimaeras in which all haematopoietic cells are derived from cathepsin D deficient mice.

RESULTS

Cathepsin D deficient bone marrow cells fully reconstitute the major cellular components of both the adaptive and innate immune systems. Spleen cells from cathepsin D deficient chimaeric mice contained an increased number of autofluorescent granules characteristic of lipofuscin positive lysosomal storage diseases. Biochemical and ultrastructural changes in cathepsin D deficient spleen are consistent with increased autolysosomal activity. Chimaeric mice were immunised with either soluble (dinitrophenylated bovine gamma globulin) or particulate (sheep red blood cells) antigens. Both antigens induced equivalent immune responses in wild type or cathepsin D deficient chimaeras.

CONCLUSION

All the parameters of haematopoietic reconstitution and adaptive immunity which were measured in this study were found to be normal in the absence of cathepsin D, even though cathepsin D deficiency leads to dysregulation of lysosomal function.

In vivo MRI reveals the dynamics of pathological changes in the brains of cathepsin D-deficient mice and correlates changes in manganese-enhanced MRI with microglial activation

Cathepsin D (CTSD; EC 3.4.23.5) is essential for normal development and/or maintenance of neurons in the central nervous system: its deficiency causes a devastating neurological disorder with severely shortened life span in man, sheep and mouse. Neuropathologically, the CTSD deficiencies are characterized by selective neuronal degeneration, gliosis and accumulation of autofluorescent proteinaceous storage material in neurons. Our aim was to study the dynamics behind the pathological alterations occurring in the brains of CTSD-deficient (CTSD-/-) mice by using in vivo magnetic resonance imaging (MRI) and histology. In order to do this, we measured T(2) signal intensity (SI), apparent diffusion coefficient, area and volume of multiple brain structures from MR images acquired using T(2)-, T(1)- and diffusion-weighted sequences at three time points during disease progression. MRI revealed no differences in the brains between CTSD-/- and control mice at postnatal day 15+/-1 (P15+/-1), representing an initial stage of the disease. In the intermediate stage of the disease, P19(+/-1), SI alterations in the thalami of the affected mice became evident in both T(1)- and T(2)-weighted images. The terminal stage of the disease, P25, was characterized by marked alterations in the T(2) SI, apparent diffusion coefficient and volume of multiple brain structures in CTSD-/- mice. In addition, manganese enhanced high-resolution T(1)-weighted 3D sequences (MEMRI) and histological stainings revealed that the hyperintense signal areas in MEMRI matched perfectly with areas of microglial activation in the brains of CTSD-/- mice at the terminal disease stage. In conclusion, the SI alterations in the thalami of CTSD-/- mice preceded other changes, and the degenerative process was greatly enhanced at the age P19(+/-1), leading to severely reduced brain volume in just 6 days.

Lysosomal proteomics and disease

A recent trend in proteomic studies has been to analyze macromolecular complexes such as subcellular organelles instead of complete cells or tissues. This "divide and conquer" approach circumvents some of the formidable problems associated with whole proteome analyses and allows focus on a subset of proteins that may be involved in a particular process or disease of interest. One organelle that has been the focus of considerable attention in proteomic studies is the lysosome, an acidic, membrane-delimited compartment that plays an essential role in the degradation and recycling of biological macromolecules. Lysosomal proteomics have been driven in part by the well-established involvement of this organelle in numerous human diseases, but also by the availability of approaches to selectively visualize and/or isolate subsets of lysosomal proteins. In terms of clinical application, proteomic studies of the lysosome have led to the identification of gene defects in three human hereditary diseases. This review summarizes past progress, current limitations and future directions in the field of lysosomal proteomics.

Proteomic analysis of cathepsin B- and L-deficient mouse brain lysosomes

Cathepsins B and L are lysosomal cysteine proteases which have been implicated in a variety of pathological processes such as cancer, tumor angiogenesis, and neurodegeneration. However, only a few protein substrates have thus far been described and the mechanisms by which cathepsins B and L regulate cell proliferation, invasion, and apoptosis are poorly understood. Combined deficiency of both cathepsins results in early-onset neurodegeneration in mice reminiscent of neuronal ceroid lipofuscinoses in humans. Therefore, we intended to quantify accumulated proteins in brain lysosomes of double deficient mice. A combination of subcellular fractionation and LC-MS/MS using isobaric tagging for relative and absolute quantitation (iTRAQ) allowed us to simultaneously assess wildtype and cathepsin B(-/-)L(-/-) cerebral lysosomes. Altogether, 19 different proteins were significantly increased in cathepsin B(-/-)L(-/-) lysosomes. Most elevated proteins had previously been localized to neuronal biosynthetic, recycling/endocytic or lysosomal compartments. A more than 10-fold increase was observed for Rab14, the Delta/Notch-like epidermal growth factor-related receptor (DNER), calcyon, and carboxypeptidase E. Intriguingly, immunohistochemistry demonstrated that Rab14 and DNER specifically stain swollen axons in double deficient brains. Since dense accumulations of expanded axons are the earliest phenotypic and pathognomonic feature of cathepsin B(-/-)L(-/-) brains, our data suggest a role for cathepsins B and L in recycling processes during axon outgrowth and synapse formation in the developing postnatal central nervous system.

Involvement of lysosomal storage-induced p38 MAP kinase activation in the overproduction of nitric oxide by microglia in cathepsin D-deficient mice

Nitric oxide (NO) and peroxynitrite, which are produced by activated microglia, are responsible for accelerated neurodegeneration in cathepsin D-deficient (CD-/-) mice. To elucidate the mechanisms by which microglia are initially activated in CD-/- mice, we analyzed the possible relationship between lysosomal storage and microglial activation. In CD-/- mice, the microglial NO-generating activity that was closely associated with the induction of inducible NO synthase and the cationic amino acid transporter-2 (CAT-2) coincided well with the lysosomal storage of subunit c of mitochondrial F0F1ATPase and the formation of ceroid/lipofuscin. Furthermore, activated microglia, which are often accumulating subunit c and ceroid/lipofuscin, showed proliferation activity and an activation of p38 mitogen-activated protein (MAP) kinase. In the primary cultured microglia, pepstatin A was found to enhance the generation of NO and superoxide anion radicals. In these pepstatin A-treated microglia, both an increased generation of the intracellular reactive oxygen species (ROS) and an activation of p38 MAP kinase were observed. These results suggest that the ceroid/lipofuscin which form in microglia activate the p38 MAP kinase cascade through the increased intracellular generation of ROS in CD-/- mice. The activated p38 MAP kinase cascade then promotes the expression of iNOS and CAT-2, thereby inducing the overproduction of NO.

A novel role of the Batten disease gene CLN3: association with BMP synthesis

Juvenile Neuronal Ceroid Lipofuscinosis (JNCL) results from a deficiency of CLN3, a protein recently identified within detergent-resistant membranes (DRMs). To study the function of CLN3 within these domains we isolated DRMs from control and JNCL-brain and noted that JNCL-derived DRMs are less buoyant than control. Analysis of DRM phospholipids derived from JNCL-brain revealed a reduction of bis(monoacylglycerol)phosphate. Metabolic labeling of JNCL-fibroblasts demonstrated a reduction in the synthesis of bis(monoacylglycerol)phosphate which was restored following complementation with wild-type-CLN3, substantiating our initial observation in brain. Metabolic labeling of cell lines overexpressing wild-type-CLN3 resulted in increased bis(monoacylglycerol)phosphate synthesis, while overexpression of mutant CLN3-L170P decreased bis(monoacylglycerol)phosphate synthesis. These data illustrate a new finding, a strong correlation between CLN3 protein expression and synthesis of bis(monoacylglycerol)phosphate.

Cathepsin D deficiency induces persistent neurodegeneration in the absence of Bax-dependent apoptosis

Neuronal ceroid lipofuscinosces/Batten disease (NCL) is a devastating group of neurodegenerative diseases caused by genetic disruptions in lysosomal function. Cathepsin D (CD) is a major lysosomal protease, and mutations in CD that render it enzymatically defective have been reported recently in subsets of NCL patients. The targeted deletion of CD in mice results in extensive neuropathology, including biochemical and morphological evidence of apoptosis and autophagic stress (aberrant autophagosome accumulation), effects that are similar to those observed in NCL. To determine the contribution of Bax-dependent apoptosis in this mouse model of NCL, combined Bax- and CD-deficient mice were generated. Morphological analysis of CD-deficient mouse brains indicated large numbers of pyknotic neurons and neurons with marked cytoplasmic swellings containing undigested lipofuscin. Cell death and apoptosis were evidenced by increases in terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) reactivity and activation of caspase-3, respectively. DeOlmos silver-positive neurons were abundant in CD-deficient brain and correlated with neuron loss, as indicated by significant decreases in NeuN (neuronal nuclear antigen)-positive neurons. Lysosome dysfunction and autophagic stress were apparent in CD-deficient brain as indicated by the accumulation of autofluorescent storage material and by increased levels of LC3-II (light chain 3-II, a selective autophagosome marker), respectively. Bax deletion significantly inhibited caspase-3 activation and hippocampal TUNEL reactivity but did not prevent the majority of CD deficiency-induced neuropathology, including the persistence of pyknotic neurons, elevated cortical TUNEL reactivity, lysosome dysfunction and autophagic stress, neurodegeneration, and neuron loss. Together, these results suggest that CD deficiency-induced neuropathology does not require Bax-dependent apoptosis and highlights the importance of caspase-independent neuron death and neurodegeneration resulting from the genetic disruption of lysosome function.

Activation of microglia acidifies lysosomes and leads to degradation of Alzheimer amyloid fibrils

Microglia are the main immune cells of the brain, and under some circumstances they can play an important role in removal of fibrillar Alzheimer amyloid beta peptide (fAbeta). Primary mouse microglia can internalize fAbeta, but they do not degrade it efficiently. We compared the level of lysosomal proteases in microglia and J774 macrophages, which can degrade fAbeta efficiently, and we found that microglia actually contain higher levels of many lysosomal proteases than macrophages. However, the microglial lysosomes are less acidic (average pH of approximately 6), reducing the activity of lysosomal enzymes in the cells. Proinflammatory treatments with macrophage colony-stimulating factor (MCSF) or interleukin-6 acidify the lysosomes of microglia and enable them to degrade fAbeta. After treatment with MCSF, the pH of microglial lysosomes is similar to J774 macrophages (pH of approximately 5), and the MCSF-induced acidification can be partially reversed upon treatment with an inhibitor of protein kinase A or with an anion transport inhibitor. Microglia also degrade fAbeta if lysosomes are acidified by an ammonia pulse-wash or by treatment with forskolin, which activates protein kinase A. Our results indicate that regulated lysosomal acidification can potentiate fAbeta degradation by microglia.

Cathepsin E Deficiency Induces a Novel Form of Lysosomal Storage Disorder Showing the Accumulation of Lysosomal Membrane Sialoglycoproteins and the Elevation of Lysosomal pH in Macrophages

Cathepsin E, an endolysosomal aspartic proteinase predominantly expressed in cells of the immune system, has an important role in immune responses. However, little is known about the precise roles of cathepsin E in this system. Here we report that cathepsin E deficiency (CatE(-/-)) leads to a novel form of lysosome storage disorder in macrophages, exhibiting the accumulation of the two major lysosomal membrane sialoglycoproteins LAMP-1 and LAMP-2 and the elevation of lysosomal pH. These striking features were also found in wild-type macrophages treated with pepstatin A and Ascaris inhibitor. Whereas there were no obvious differences in their expression, biosynthesis, and trafficking between wild-type and CatE(-/-) macrophages, the degradation rates of these two membrane proteins were apparently decreased as a result of cathepsin E deficiency. Because there was no difference in the vacuolar-type H(+)-ATPase activity in both cell types, the elevated lysosomal pH in CatE(-/-) macrophages is most likely due to the accumulation of these lysosomal membrane glycoproteins highly modified with acidic monosaccharides, thereby leading to the disruption of non-proton factors controlling lysosomal pH. Furthermore, the selective degradation of LAMP-1 and LAMP-2, as well as LIMP-2, was also observed by treatment of the lysosomal membrane fraction isolated from wild-type macrophages with purified cathepsin E at pH 5. Our results thus suggest that cathepsin E is important for preventing the accumulation of these lysosomal membrane sialoglycoproteins that can induce a new form of lysosomal storage disorder.

Cathepsin D expression is decreased in Alzheimer's disease fibroblasts

Cathepsin D (CTSD), a protease detectable in different cell types whose primary function is to degrade proteins by bulk proteolysis in lysosomes, has been suggested to be involved in Alzheimer's disease (AD). In fact, there is increasing evidence that disturbance of the normal balance and localization of cathepsins may contribute to neurodegeneration in AD [Nakanishi H. Neuronal and microglial cathepsins in aging and age-related diseases. Aging Res Rev 2003; 2(4):367-81]. Here, we provide evidence of an altered balance of CTSD in skin fibroblasts from patients affected either by sporadic or familial forms of AD. In particular, we demonstrate that CTSD is down regulated at both transcriptional and translational level and its processing is altered in AD fibroblasts. The oncogene Ras is involved in the regulation of CTSD, as high expression level of the constitutively active form of Ras in normal or AD fibroblasts induces CTSD down-regulation. p38 MAPK signalling pathway also appears to down-modulate CTSD level. Overall results reinforce the hypothesis that a lysosomal impairment may be involved in AD pathogenesis and can be detected not only in the CNS but also at a peripheral level.

Functional biology of the neuronal ceroid lipofuscinoses (NCL) proteins

Neuronal ceroid lipofucinoses (NCLs) are a group of severe neurodegenerative disorders characterized by accumulation of autofluorescent ceroid lipopigment in patients' cells. The different forms of NCL share many similar pathological features but result from mutations in different genes. The genes affected in NCLs encode both soluble and transmembrane proteins and are localized to ER or to the endosomes/lysosomes. Due to selective vulnerability of the central nervous system in the NCL disorders, the corresponding proteins are proposed to have important, tissue specific roles in the brain. The pathological similarities of the different NCLs have led not only to the grouping of these disorders but also to suggestion that the NCL proteins function in the same biological pathway. Despite extensive research, including the development of several model organisms for NCLs and establishment of high-throughput techniques, the precise biological function of many of the NCL proteins has remained elusive. The aim of this review is to summarize the current knowledge of the functions, or proposed functions, of the different NCL proteins.

The neuronal ceroid-lipofuscinoses: From past to present

The neuronal ceroid-lipofuscinoses (NCLs) are inherited lysosomal storage diseases and constitute the most common group of children's progressive encephalopathies. Most childhood forms of NCL are clinically characterized by progressive loss of vision as well as mental and motor deterioration, epileptic seizures, and premature death, while the rare adult forms are dominated by dementia. All forms of NCL share common pathomorphological features. Autofluorescent, periodic acid-Schiff- and Sudan black B-positive granules, resistant to lipid solvents, accumulate in the cytoplasm of most nerve cells, and there is progressive and remarkably selective neuronal degeneration and loss. For a long time, the NCLs were grouped under the heading of the "amaurotic family idiocies" and conceived as lipidoses. However, in the late 1980s and 1990s the NCL storage cytosomes were shown to consist largely of two hydrophobic proteins: either subunit c of mitochondrial ATP synthase or sphingolipid activator proteins A and D. Since 1995 numerous mutations in at least seven different genes have been shown to underlie the multiple human and animal forms of NCL. This review discusses the historical evolution of the NCL concept and the impact of the recent biochemical and molecular genetic findings on our views on the classification and pathogenesis of these devastating brain disorders.

Progress towards understanding disease mechanisms in small vertebrate models of neuronal ceroid lipofuscinosis

Model systems provide an invaluable tool for investigating the molecular mechanisms underlying the NCLs, devastating neurodegenerative disorders that affect the relatively inaccessible tissues of the central nervous system. These models have enabled the assessment of behavioural, pathological, cellular, and molecular abnormalities, and also allow for development and evaluation of novel therapies. This review highlights the relative advantages of the two available small vertebrate species, the mouse and zebrafish, in modelling NCL disease, summarising how these have been useful in NCL research and their potential for the development and testing of prospective disease treatments. A panel of mouse mutants is available representing all the cloned NCL gene disorders (Cathepsin D, CLN1, CLN2, CLN3, CLN5, CLN6, CLN8). These NCL mice all have progressive neurodegenerative phenotypes that closely resemble the pathology of human NCL. The analysis of these models has highlighted several novel aspects underlying NCL pathogenesis including the selective nature of neurodegeneration, evidence for glial responses that precede neuronal loss and identification of the thalamus as an important pathological target early in disease progression. Studies in mice have also highlighted an unexpected heterogeneity underlying NCL phenotypes, and novel potential NCL-like mouse models have been described including mice with mutations in cathepsins, CLC chloride channels, and other lysosome-related genes. These new models are likely to provide significant new information on the spectrum of NCL disease. Information on NCL mice is available in the NCL Mouse Model Database (). There are homologs of most of the NCL genes in zebrafish, and NCL zebrafish models are currently in development. This model system provides additional advantages to those provided by NCL mouse models including high-throughput mutational, pharmacogenetic and therapeutic technique analyses. Mouse and zebrafish models are an important shared resource for NCL research, offering a unique possibility to dissect disease mechanisms and to develop therapeutic approaches.

A missense mutation (c.184C>T) in ovine CLN6 causes neuronal ceroid lipofuscinosis in Merino sheep whereas affected South Hampshire sheep have reduced levels of CLN6 mRNA

The neuronal ceroid lipofuscinoses (NCLs, Batten disease) are a group of fatal recessively inherited neurodegenerative diseases of humans and animals characterised by common clinical signs and pathology. These include blindness, ataxia, dementia, behavioural changes, seizures, brain and retinal atrophy and accumulation of fluorescent lysosome derived organelles in most cells. A number of different variants have been suggested and seven different causative genes identified in humans (CLN1, CLN2, CLN3, CLN5, CLN6, CLN8 and CTSD). Animal models have played a central role in the investigation of this group of diseases and are extremely valuable for developing a better understanding of the disease mechanisms and possible therapeutic approaches. Ovine models include flocks of affected New Zealand South Hampshires and Borderdales and Australian Merinos. The ovine CLN6 gene has been sequenced in a representative selection of these sheep. These investigations unveiled the mutation responsible for the disease in Merino sheep (c.184C>T; p.Arg62Cys) and three common ovine allelic variants (c.56A>G, c.822G>A and c.933_934insCT). Linkage analysis established that CLN6 is the gene most likely to cause NCL in affected South Hampshire sheep, which do not have the c.184C>T mutation but show reduced expression of CLN6 mRNA in a range of tissues as determined by real-time PCR. Lack of linkage precludes CLN6 as a candidate for NCL in Borderdale sheep.

Lysosomal storage disease upon disruption of the neuronal chloride transport protein ClC-6

Mammalian CLC proteins function as Cl(-) channels or as electrogenic Cl(-)/H(+) exchangers and are present in the plasma membrane and intracellular vesicles. We now show that the ClC-6 protein is almost exclusively expressed in neurons of the central and peripheral nervous systems, with a particularly high expression in dorsal root ganglia. ClC-6 colocalized with markers for late endosomes in neuronal cell bodies. The disruption of ClC-6 in mice reduced their pain sensitivity and caused moderate behavioral abnormalities. Neuronal tissues showed autofluorescence at initial axon segments. At these sites, electron microscopy revealed electron-dense storage material that caused a pathological enlargement of proximal axons. These deposits were positive for several lysosomal proteins and other marker proteins typical for neuronal ceroid lipofuscinosis (NCL), a lysosomal storage disease. However, the lysosomal pH of Clcn6(-/-) neurons appeared normal. CLCN6 is a candidate gene for mild forms of human NCL. Analysis of 75 NCL patients identified ClC-6 amino acid exchanges in two patients but failed to prove a causative role of CLCN6 in that disease.

Autophagy in neurodegenerative disease: friend, foe or turncoat?

Autophagy, a lysosomal pathway for degrading organelles and long-lived proteins, is becoming recognized as a key adaptive response that can preclude death in stressed or diseased cells. However, during development strong induction of autophagy in specific cell populations mediates a type of programmed cell death that has distinctive 'autophagic' morphology and a requirement for autophagy activity. The recent identification of autophagosomes in neurons in a growing number of neurodegenerative disorders has, therefore, sparked controversy about whether these structures are contributing to neuronal cell death or protecting against it. Emerging evidence supports the view that induction of autophagy is a neuroprotective response and that inadequate or defective autophagy, rather than excessive autophagy, promotes neuronal cell death in most of these disorders. In this review, we consider possible mechanisms underlying autophagy-associated cell death and their relationship to pathways mediating apoptosis and necrosis.

Cathepsin D deficiency is associated with a human neurodegenerative disorder

Cathepsin D is a ubiquitously expressed lysosomal protease that is involved in proteolytic degradation, cell invasion, and apoptosis. In mice and sheep, cathepsin D deficiency is known to cause a fatal neurodegenerative disease. Here, we report a novel disorder in a child with early blindness and progressive psychomotor disability. Two missense mutations in the CTSD gene, F229I and W383C, were identified and were found to cause markedly reduced proteolytic activity and a diminished amount of cathepsin D in patient fibroblasts. Expression of cathepsin D mutants in cathepsin D(-/-) mouse fibroblasts revealed disturbed posttranslational processing and intracellular targeting for W383C and diminished maximal enzyme velocity for F229I. The structural effects of cathepsin D mutants were estimated by computer modeling, which suggested larger structural alterations for W383C than for F229I. Our studies broaden the group of human neurodegenerative disorders and add new insight into the cellular functions of human cathepsin D.

Molecular genetics of the NCLs -- status and perspectives

The neuronal ceroid lipofuscinoses (NCLs) are a group of inherited neurodegenerative disorders characterized by the accumulation of autofluorescent storage material in many cell types, including neurons. Most NCL subtypes are inherited in an autosomal recessive manner and characterized clinically by epileptic seizures, progressive psychomotor decline, visual failure, variable age of onset, and premature death. To date, seven genes underlying human NCLs have been identified. Most of the mutations in these genes are associated with specific disease subtypes, while some result in variable disease onset, severity and progression. In addition to these, there are still disease subgroups with unknown molecular genetic backgrounds. Although apparent clinical homogeneity exists within some of these subgroups, actual genetic heterogeneity may complicate gene identification. Additional clues to the identification of these unknown genes may come from animal models of NCL and from functional studies of already known genes which may suggest further candidates.

A mutation in the cathepsin D gene (CTSD) in American Bulldogs with neuronal ceroid lipofuscinosis

We obtained DNA, brains, and eyes from American Bulldogs with neurodegeneration due to neuronal ceroid lipofuscinosis (NCL). The diagnosis of NCL was confirmed by detection of autofluorescent cytoplasmic inclusions within neurons throughout the brains, in retinal ganglion cells, and along outer limiting membranes of the retinas. Electron microscopy revealed that the inclusions had coarsely granular matrices surrounding well-delineated spherical structures and that the inclusions near the retinal outer limiting membranes were within photoreceptor cells, mostly cones. Affected American Bulldogs were homozygous for the A allele of a G to A transition in the cathepsin D gene (CTSD), which predicts the conversion of methionine-199 to an isoleucine. Only the G allele was detected in DNA samples from 131 randomly selected dogs representing 108 breeds other than American Bulldog; however, the A allele had a frequency of 0.28 among 123 genotyped American Bulldogs. Transmission analysis in a 99 dog pedigree of American Bulldogs indicated a probability of less than 10(-7) that alleles from any mutation unlinked to CTSD would be concordant with the pedigree and phenotypes of the dogs. Brain samples from affected dogs had 36% of the cathepsin D-specific enzymatic activity found in control dog brains; whereas, specific enzymatic activities of 15 other lysosomal enzymes were unchanged or increased. Compared to previously described NCLs in mice and sheep that completely lack cathepsin D activity, the clinical course of NCL in the American Bulldogs was less severe and more closely resembled that of many human NCLs.

You say lipofuscin, we say ceroid: defining autofluorescent storage material

Accumulation of intracellular autofluorescent material or "aging pigment" has been characterized as a normal aging event. Certain diseases also exhibit a similar accumulation of intracellular autofluorescent material. However, autofluorescent storage material associated with aging and disease has distinct characteristics. Lipofuscin is a common term for aging pigments, whereas ceroid is used to describe pathologically derived storage material, for example, in the neuronal ceroid lipofuscinoses (NCLs). NCLs are a family of neurodegenerative diseases that are characterized by an accumulation of autofluorescent storage material (ceroid) in the lysosome, which has been termed "lipofuscin-like". There have been many studies that describe this autofluorescent storage material, but what is it? Is this accumulation lipofuscin or ceroid? In this review we will try to answer the following questions: (1) What is lipofuscin and ceroid? (2) What contributes to the accumulation of this storage material in one or the other? (3) Does this material have an effect on cellular function? Studying parallels between the accumulation of lipofuscin and ceroid may provide insight into the biological relevance of these phenomena.

Influence and interactions of cathepsin D, HLA-DRB1 and APOE on cognitive abilities in an older non-demented population

Cathepsin D (CTSD), human leukocyte antigen DRB1 (HLA-DRB1) and apolipoprotein E (APOE) have all been associated with cognitive ability in both demented and non-demented individuals. CTSD is a pleiotrophic protein whose functions include the processing of proteins prior to presentation by HLA. Several studies have also reported that a functional exon 2 polymorphism in the CTSD gene interacts with APOEepsilon4 resulting in an increased risk of developing Alzheimer's disease (AD). We have previously reported that the CTSD exon 2 polymorphism regulates fluid intelligence. In this study, we extend this finding to other cognitive domains and investigate interactions with APOE and HLA-DRB1. Using a cohort of 766 non-demented volunteers, we found that the CTSD exon 2 T allele was associated with a decrease in several cognitive domains that comprise processing speed [random letters (RLs) test, P = 0.012; alphabet-coding task (ACT), P = 0.001], spatial recall (SR) (P = 0.016) and an additional test of fluid intelligence (P = 0.010). We also observed that the HLA-DR1 was associated with enhanced cumulative recall ability (P = 0.006), and conversely HLA-DR5 was associated with diminished delayed verbal recall and SR abilities (P = 0.014 and P = 0.003, respectively). When analysed independently, APOEepsilon4 did not influence any cognitive domains. In contrast, CTSD T/APOEepsilon4-positive volunteers scored lower on tests of fluid intelligence (P = 0.015), processing speed (ACT, P = 0.001; RL, P = 0.013) and immediate recall (P = 0.029). Scores were lower for all these tests than when CTSD and APOE were analysed independently. This supports previous findings in AD that have also reported an epistatic interaction. In addition, we found that CTSD T/HLA-DR2-positive volunteers had reduced processing speed (ACT, P = 0.040; RL, P = 0.014) and had significantly lower cumulative and SR abilities (P = 0.003 and P = 0.001, respectively). Biological interaction between these two proteins has previously been shown where HLA-DR2 binds more readily to the myelin basic protein (MBP) compared with other DR antigens, preventing MBP cleavage by CTSD.

Overexpression in colorectal carcinoma of two lysosomal enzymes, CLN2 and CLN1, involved in neuronal ceroid lipofuscinosis

BACKGROUND

Lysosomal proteases are implicated in cancer progression and metastasis. In the current study, using subtraction cloning for genes that are differentially expressed in metastasis, the authors isolated a clone encoding ceroid lipofuscinosis, neuronal 2 (CLN2), which is a lysosomal serine protease defective in neuronal ceroid lipofuscinosis (NCL). Increased CLN2 activity has been reported in breast carcinoma and the antiapoptotic effect of another causative gene of NCL, ceroid lipofuscinosis, neuronal 1 (CLN1), is known.

METHODS

The mRNA levels of CLN2, CLN1, and cathepsins B, D, H, and L were investigated in colorectal carcinoma patients with different clinical stages using real-time quantitative reverse transcriptase polymerase chain reaction. A polyclonal antibody was raised against a recombinant CLN2 protein for immunoblotting and immunohistochemistry.

RESULTS

The mRNA levels of CLN1 and cathepsins B, D, and L were significantly higher in metastatic lesions than in primary tumors. In the primary tumors, mRNA expressions of CLN2 and cathepsin D were associated with advanced clinical stages (P < .015 and P < .031, respectively). Among the lysosomal enzymes examined, only the mRNA expression of CLN2 in both the primary tumors of all patients and the pT3 tumors was correlated with the presence of liver metastases (P < .0049 and P < .029, respectively). The polyclonal antibody prepared in the current study demonstrated CLN2 overexpression by immunoblotting and immunohistochemistry.

CONCLUSIONS

The results indicate that there is a close correlation between CLN2 and CLN1 expression and colorectal carcinoma progression and metastasis and suggest that they may be potential molecular targets.

Lysosomes and lysosomal proteins in cancer cell death (new players of an old struggle)

Death of cancer cells influences tumor development and progression, as well as the response to anticancer therapies. This can occur through different cell death programmes which have recently been shown to implicate components of the acidic organelles, lysosomes. The role of lysosomes and lysosomal enzymes, including cathepsins and some lipid hydrolases, in programmed cell death associated with apoptotic or autophagic phenotypes is presented, as evidenced from observations on cultured cells and living animals. The possible molecular mechanisms that underlie the action of lysosomes during cell death are also described. Finally, the contribution of lysosomal proteins and lysosomes to tumor initiation and progression is discussed. Elucidation of this role and the underlying mechanisms will shed a new light on these 'old' organelles and hopefully pave the way for the development of novel anticancer strategies.

Neuronal ceroid-lipofuscinosis in Borderdale sheep

AIM

To describe the gross and histological lesions of a neurological disease in Borderdale sheep characterised clinically by blindness and circling, as a basis to its classification.

METHODS

Formalin-fixed tissues were processed into paraffin wax and epoxy resin for light and electron microscopy of variously stained sections.

RESULTS

Lesions were those of a lysosomal storage disease with severe neurodegeneration of the cerebral cortex. The staining reactions, autofluorescence and ultrastructure of storage material allowed the diagnosis of neuronal ceroid-lipofuscinosis associated with the accumulation of subunit-c of mitochondrial ATP synthase.

CONCLUSIONS

The severity of neurodegeneration and minor differences in the ultrastructure of storage material implied that this was a different disease from other forms of ovine ceroid-lipofuscinosis that accumulate subunit-c of mitochondrial ATP synthase. An autosomal recessive mode of inheritance is considered probable. Although of only minor economic importance, this disease may be important to research into the group of ceroid-lipofuscinoses as a whole.

Two novel CLN6 mutations in variant late-infantile neuronal ceroid lipofuscinosis patients of Turkish origin

Neuronal ceroid lipofuscinoses (NCLs) are the most common neurodegenerative childhood-onset disorders characterized by autosomal recessive inheritance, epileptic seizures, progressive psychomotor deterioration, visual failure, and premature death. At least seven subtypes of childhood-onset NCLs have been identified of which the late-infantile-onset forms (LINCLs) are genetically the most heterogeneous with four underlying genes identified. A variant form of LINCL (vLINCL) present in Turkish patients has been considered a distinct clinical and genetic entity (CLN7). However, we recently showed that mutations in the CLN8 gene account for a subset of Turkish vLINCL. Toward identifying the CLN7 gene we here screened the known NCL loci for homozygosity in nine Turkish vLINCL families. These loci were excluded in seven families that are likely to represent the 'true' Turkish vLINCL. In two families, we identified two novel homozygous mutations in the CLN6 gene: an intronic base substitution (c.542+5G>T) affecting the splicing of the transcript and a nonsense mutation (c.663C>G) creating a stop codon at tyrosine 221. These data indicate that CLN6 mutations, in addition to those of CLN8, should be considered a diagnostic alternative in Turkish vLINCL patients. The genetic background of the 'true' Turkish vLINCL, CLN7, remains to be defined.

Correlations between genotype, ultrastructural morphology and clinical phenotype in the neuronal ceroid lipofuscinoses

The neuronal ceroid lipofuscinoses (NCLs) are a group of severe neurodegenerative diseases with onset usually in childhood and characterised by the intracellular accumulation of autofluorescent storage material. Within the last decade, mutations that cause NCL have been found in six human genes (CLN1, CLN2, CLN3, CLN5, CLN6 and CLN8). Mutations in two additional genes cause disease in animal models that share features with NCL-CTSD in sheep and mice and PPT2 in mice. Approximately 160 NCL disease-causing mutations have now been described (listed and fully cited in the NCL Mutation Database, http://www.ucl.ac.uk/ncl/ ). Most mutations result in a classic morphology and disease phenotype, but some mutations are associated with disease that is of later onset, less severe or protracted in its course, or with atypical morphology. Seven common mutations exist, some having a worldwide distribution and others associated with families originating from specific geographical regions. This review attempts to correlate the gene, disease-causing mutation, morphology and clinical phenotype for each type of NCL.

High yield synthesis and characterization of phosphorylated recombinant human procathepsin D expressed in mammalian cells

We used a vaccinia virus expression system for the production of recombinant human cathepsin D (CD), a lysosomal protease implicated in various patho-physiological processes including cancer, neurodegeneration, and development. The recombinant protein was successfully expressed in various human and non-human cells. It was correctly synthesized as a glycosylated 53 kDa precursor (proCDrec) that reacted with a polyclonal antibody against residues 7-21 of the propeptide sequence. In contrast to the control, in cells infected with the recombinant virus proCDrec was largely secreted into the culture medium, although it contained high-mannose oligosaccharides with uncovered mannose-6-phosphate residues. Intracellular proCDrec was processed into the 48 kDa intermediate single-chain and the 31 plus 13 kDa double-chain forms, however, the processing was slower than in normal cells. A method based on Pepstatin A-affinity chromatography allowed to isolate the recombinant protein from the medium of infected cells. Based on its latency in activity assay at acid pH and on its reactivity with antibodies specific for the N-terminus, the purified protein was judged to be in the inactive precursor form. During incubation at acid pH the purified proCDrec underwent autocatalytic processing and acquired pepstatin A-sensitive enzyme activity, as expected for correctly folded proCD. Antiserum raised in rabbits against proCDrec specifically reacted with human, but not with mouse proCD under non-denaturing conditions. We conclude that our vaccinia virus-directed proCDrec displays structural and functional features resembling those of native human proCD. This system can therefore be exploited for the synthesis of large quantities of human proCD, allowing further studies on the structure and function of this interesting protein.

Medical bioremediation: prospects for the application of microbial catabolic diversity to aging and several major age-related diseases

Several major diseases of old age, including atherosclerosis, macular degeneration and neurodegenerative diseases are associated with the intracellular accumulation of substances that impair cellular function and viability. Moreover, the accumulation of lipofuscin, a substance that may have similarly deleterious effects, is one of the most universal markers of aging in postmitotic cells. Reversing this accumulation may thus be valuable, but has proven challenging, doubtless because substances resistant to cellular catabolism are inherently hard to degrade. We suggest a radically new approach: augmenting humans' natural catabolic machinery with microbial enzymes. Many recalcitrant organic molecules are naturally degraded in the soil. Since the soil in certain environments - graveyards, for example - is enriched in human remains but does not accumulate these substances, it presumably harbours microbes that degrade them. The enzymes responsible could be identified and engineered to metabolise these substances in vivo. Here, we survey a range of such substances, their putative roles in age-related diseases and the possible benefits of their removal. We discuss how microbes capable of degrading them can be isolated, characterised and their relevant enzymes engineered for this purpose and ways to avoid potential side-effects.

Cathepsin D-deficient Drosophila recapitulate the key features of neuronal ceroid lipofuscinoses

Neuronal ceroid lipofuscinoses (NCLs) are a group of lysosomal storage disorders characterized pathologically by neuronal accumulation of autofluorescent storage material and neurodegeneration. An ovine NCL form is caused by a recessive point mutation in the cathepsin D gene, which encodes a lysosomal aspartyl protease. This mutation results in typical NCL pathology with neurodegeneration and characteristic neuronal storage material. We have generated a Drosophila NCL model by inactivating the conserved Drosophila cathepsin D homolog. We report here that cathepsin D mutant flies exhibit the key features of NCLs. They show progressive neuronal accumulation of autofluorescent storage inclusions, which are also positive for periodic acid Schiff and luxol fast blue stains. Ultrastructurally, the storage material is composed of membrane-bound granular electron-dense material, similar to the granular osmiophilic deposits found in the human infantile and ovine congenital NCL forms. In addition, cathepsin D mutant flies show modest age-dependent neurodegeneration. Our results suggest that the metabolic pathway leading to NCL pathology is highly conserved during evolution, and that cathepsin D mutant flies can be used to study the pathogenesis of NCLs.

DNA degradation in development and programmed cell death

Most mammalian cells have nuclei that contain DNA, which replicates during cell proliferation. DNA is destroyed by various developmental processes in mammals. It is degraded during programmed cell death that accompanies mammalian development. The nuclei of erythrocytes and eye lens fiber cells are also removed during their differentiation into mature cells. If DNA is not properly degraded in these processes, it can cause various diseases, including tissue atrophy, anemia, cataract, and autoimmune diseases, which indicates that DNA can be a pathogenic molecule. Here, I present how DNA is degraded during programmed cell death, erythroid cell differentiation, and lens cell differentiation. I discuss what might be or will be learned from understanding the molecular mechanisms of DNA degradation that occurs during mammalian development.

Characterization of candidate genes for neuronal ceroid lipofuscinosis in dog

The neuronal ceroid lipofuscinoses (NCL) are a heterogenous group of monogenic autosomal recessive inherited progressive neurodegenerative diseases characterized by brain and retinal atrophy and the intracellular accumulation of autofluorescent lysosomal storage bodies resembling lipofuscin in neurons and other cells. Until today, eight forms of NCL have been classified in humans by clinical criteria, which result from mutations in at least six different genes (TPP1, CLN2, PPT1, CLN5, CLN6, and CLN8). NCL has also been reported in various domestic animal species including cattle, goat, sheep, cat, and certain dog breeds. In this report, the experimental analysis of canine PPT1, CLN5, CLN6, and CLN8 full-length cDNA sequences is described, and the current whole genome sequence assembly was used for gene structure analyses. Characterization of the four canine genes revealed a conserved organization with respect to the human orthologs. In general the gene size in dog is smaller compared to the human sequence due to shorter intron length. Using four individuals of Tibetan terrier with NCL, and a single affected Polish Owczarek Nizinny (PON) dog, we excluded the complete coding region of canine PPT1 and CLN8 and three of four exons of CLN5 and six of seven exons of CLN6 harboring disease-causing mutations.

The neuronal ceroid lipofuscinosis Cln8 gene expression is developmentally regulated in mouse brain and up-regulated in the hippocampal kindling model of epilepsy

BACKGROUND

The neuronal ceroid lipofuscinoses (NCLs) are a group of inherited neurodegenerative disorders characterized by accumulation of autofluorescent material in many tissues, especially in neurons. Mutations in the CLN8 gene, encoding an endoplasmic reticulum (ER) transmembrane protein of unknown function, underlie NCL phenotypes in humans and mice. The human phenotype is characterized by epilepsy, progressive psychomotor deterioration and visual loss, while motor neuron degeneration (mnd) mice with a Cln8 mutation show progressive motor neuron dysfunction and retinal degeneration.

RESULTS

We investigated spatial and temporal expression of Cln8 messenger ribonucleic acid (mRNA) using in situ hybridization, reverse transcriptase polymerase chain reaction (RT-PCR) and northern blotting. Cln8 is ubiquitously expressed at low levels in embryonic and adult tissues. In prenatal embryos Cln8 is most prominently expressed in the developing gastrointestinal tract, dorsal root ganglia (DRG) and brain. In postnatal brain the highest expression is in the cortex and hippocampus. Expression of Cln8 mRNA in the central nervous system (CNS) was also analyzed in the hippocampal electrical kindling model of epilepsy, in which Cln8 expression was rapidly up-regulated in hippocampal pyramidal and granular neurons.

CONCLUSION

Expression of Cln8 in the developing and mature brain suggests roles for Cln8 in maturation, differentiation and supporting the survival of different neuronal populations. The relevance of Cln8 up-regulation in hippocampal neurons of kindled mice should be further explored.

Proteolytic mechanisms in necrotic cell death and neurodegeneration

Programmed neuronal cell death is required during development to achieve the accurate wiring of the nervous system. However, genetic or accidental factors can lead to the premature, non-programmed death of neurons during adult life. Inappropriate death of cells in the nervous system is the cause of multiple neurodegenerative disorders. Pathological neuronal death can occur by apoptosis, by necrosis or by a combination of both. Necrotic cell death underlies the pathology of devastating neurological diseases such as neurodegenerative disorders, stroke or trauma. However, little is known about the molecular mechanisms that bring about necrotic cell death. Proteases play crucial roles in neuron degeneration by exerting both regulatory and catabolic functions. Elevated intracellular calcium is the most ubiquitous feature of neuronal death with the concomitant activation of cysteine calcium-dependent proteases, calpains. Calpains and lysosomal, catabolic aspartyl proteases, play key roles in the necrotic death of neurons. In this review, we survey the recent literature on the role of cysteine and aspartyl proteases in necrosis and neurodegeneration, aiming to delineate common proteolytic mechanisms mediating cellular destruction.

A mutation in the CLN8 gene in English Setter dogs with neuronal ceroid-lipofuscinosis

A heritable neurodegenerative disease of English Setters has long been studied as a model of human neuronal ceroid-lipofuscinosis (NCL). Megablast searches of the first build of the canine genome for potential causative genes located the CLN8 gene near the q telomere of canine chromosome 37, close to a marker previously linked to English Setter NCL. Sequence analysis of the coding region from affected dogs revealed a T-to-C transition in the CLN8 gene that predicts a p.L164P missense mutation. Leucine 164 is conserved in four other mammalian species. The C allele co-segregated with the disease phenotype in a two-generation English Setter family in a pattern consistent with autosomal recessive inheritance. All four NCL-affected family members were C/C homozygotes and all four obligate carriers were C/T heterozygotes; whereas, 103 unrelated dogs were all T/T homozygotes. These findings indicate that the CLN8 T-to-C transition is the likely cause of English Setter NCL.

Loss of the chloride channel ClC-7 leads to lysosomal storage disease and neurodegeneration

ClC-7 is a chloride channel of late endosomes and lysosomes. In osteoclasts, it may cooperate with H(+)-ATPases in acidifying the resorption lacuna. In mice and man, loss of ClC-7 or the H(+)-ATPase a3 subunit causes osteopetrosis, a disease characterized by defective bone resorption. We show that ClC-7 knockout mice additionally display neurodegeneration and severe lysosomal storage disease despite unchanged lysosomal pH in cultured neurons. Rescuing their bone phenotype by transgenic expression of ClC-7 in osteoclasts moderately increased their lifespan and revealed a further progression of the central nervous system pathology. Histological analysis demonstrated an accumulation of electron-dense material in neurons, autofluorescent structures, microglial activation and astrogliosis. Like in human neuronal ceroid lipofuscinosis, there was a strong accumulation of subunit c of the mitochondrial ATP synthase and increased amounts of lysosomal enzymes. Such alterations were minor or absent in ClC-3 knockout mice, despite a massive neurodegeneration. Osteopetrotic oc/oc mice, lacking a functional H(+)-ATPase a3 subunit, showed no comparable retinal or neuronal degeneration. There are important medical implications as defects in the H(+)-ATPase and ClC-7 can underlie human osteopetrosis.

Mice with Ppt1Δex4 mutation replicate the INCL phenotype and show an inflammation-associated loss of interneurons

Infantile Neuronal Ceroid Lipofuscinosis (INCL) results from mutations in the palmitoyl protein thioesterase (PPT1, CLN1) gene and is characterized by dramatic death of cortical neurons. We generated Ppt1Deltaex4 mice by a targeted deletion of exon 4 of the mouse Ppt1 gene. Similar to the clinical phenotype, the homozygous mutants show loss of vision from the age of 8 weeks, seizures after 4 months and paralysis of hind limbs at the age of 5 months. Autopsy revealed a dramatic loss of brain mass and histopathology demonstrated accumulation of autofluorescent granular osmiophilic deposits (GRODS), both characteristic of INCL. At 6 months, the homozygous Ppt1Deltaex4 mice showed a prominent loss of GABAergic interneurons in several brain areas. The transcript profiles of wild-type and mutant mouse brains revealed that most prominent alterations involved parts of the immune response, implicating alterations similar to those of the aging brain and neurodegeneration. These findings make the Ppt1Deltaex4 mouse an interesting model for the inflammation-associated death of interneurons.

A Dictyostelium mutant with reduced lysozyme levels compensates by increased phagocytic activity

Lysozymes are bacteria-degrading enzymes and play a major role in the immune defense of animals. In free-living protozoa, lysozyme-like proteins are involved in the digestion of phagocytosed bacteria. Here, we purified a protein with lysozyme activity from Dictyostelium amoebae, which constitutes the founding member, a novel class of lysozymes. By tagging the protein with green fluorescent protein or the Myc epitope, a new type of lysozyme-containing vesicle was identified that was devoid of other known lysosomal enzymes. The most highly expressed isoform, encoded by the alyA gene, was knocked out by homologous recombination. The mutant cells had greatly reduced enzymatic activity and grew inefficiently when bacteria were the sole food source. Over time the mutant gained the ability to internalize bacteria more efficiently, so that the defect in digestion was compensated by increased uptake of food particles.

pdf1, a palmitoyl protein thioesterase 1 Ortholog in Schizosaccharomyces pombe: a yeast model of infantile Batten disease

Infantile Batten disease is a severe neurodegenerative storage disorder caused by mutations in the human PPT1 (palmitoyl protein thioesterase 1) gene, which encodes a lysosomal hydrolase that removes fatty acids from lipid-modified proteins. PPT1 has orthologs in many species, including lower organisms and plants, but not in Saccharomyces cerevisiae. The fission yeast Schizosaccharomyces pombe contains a previously uncharacterized open reading frame (SPBC530.12c) that encodes the S. pombe Ppt1p ortholog fused in frame to a second enzyme that is highly similar to a previously cloned mouse dolichol pyrophosphatase (Dolpp1p). In the present study, we characterized this interesting gene (designated here as pdf1, for palmitoyl protein thioesterase-dolichol pyrophosphate phosphatase fusion 1) through deletion of the open reading frame and complementation by plasmids bearing mutations in various regions of the pdf1 sequence. Strains bearing a deletion of the entire pdf1 open reading frame are nonviable and are rescued by a pdf1 expression plasmid. Inactivating mutations in the Dolpp1p domain do not rescue the lethality, whereas mutations in the Ppt1p domain result in cells that are viable but abnormally sensitive to sodium orthovanadate and elevated extracellular pH. The latter phenotypes have been previously associated with class C and class D vacuolar protein sorting (vps) mutants and vacuolar membrane H(+)-ATPase (vma) mutants in S. cerevisiae. Importantly, the Ppt1p-deficient phenotype is complemented by the human PPT1 gene. These results indicate that the function of PPT1 has been widely conserved throughout evolution and that S. pombe may serve as a genetically tractable model for the study of human infantile Batten disease.

Mannose 6-Phosphorylated Proteins Are Required for Tumor Necrosis Factor-induced Apoptosis

Whereas caspases are essential components in apoptosis, other proteases seem to be involved in programmed cell death. This study investigated the role of lysosomal mannose 6-phosphorylated proteins in tumor necrosis factor (TNF)-induced apoptosis. We report that fibroblasts isolated from patients affected with inclusion-cell disease (ICD), having a deficient activity of almost all lysosomal hydrolases, are resistant to the toxic effect of TNF. These mutant cells exhibited a defect in TNF-induced caspase activation, Bid cleavage, and release of cytochrome c. In contrast, TNF-induced p42/p44 MAPK activation and CD54 expression remained unaltered. Human ICD lymphoblasts and fibroblasts derived from mice nullizygous for Igf2 and the two mannose 6-phosphate (M6P) receptors, Mpr300 and Mpr46, which develop an ICD-like phenotype, were also resistant to CD95 ligand and TNF, respectively. Moreover, correction of the lysosomal enzyme defect of ICD fibroblasts, using a medium enriched in M6P-containing proteins, enabled restoration of sensitivity to TNF. This effect was blocked by exogenous M6P but not by cathepsin B or L inhibitors. Altogether, these findings suggest that some M6P-bearing glycoproteins modulate the susceptibility to TNF-induced apoptosis. As a matter of fact, exogenous tripeptidyl peptidase 1, a lysosomal carboxypeptidase, could sensitize ICD fibroblasts to TNF. These observations highlight the hitherto unrecognized role of some mannose 6-phosphorylated proteins such as tripeptidyl peptidase 1 in the apoptotic cascade triggered by TNF.

CLN6, which is associated with a lysosomal storage disease, is an endoplasmic reticulum protein

The neuronal ceroid lipofuscinoses (NCLs) are severe inherited neurodegenerative disorders affecting children. In this disease, lysosomes accumulate autofluorescent storage material and there is death of neurons. Five types of NCL are caused by mutations in lysosomal proteins (CTSD, CLN1/PPT1, CLN2/TTPI, CLN3 and CLN5), and one type is caused by mutations in a protein that recycles between the ER and ERGIC (CLN8). The CLN6 gene underlying a variant of late infantile NCL (vLINCL) was recently identified. It encodes a novel 311 amino acid transmembrane protein. Antisera raised against CLN6 peptides detected a protein of 30 kDa by Western blotting of human cells, which was missing in cells from some CLN6 deficient patients. Using immunofluorescence microscopy, CLN6 was shown to reside in the endoplasmic reticulum (ER). CLN6 protein tagged with GFP at the C-terminus and expressed in HEK293 cells was also found within the ER. Investigation of the effect of five CLN6 disease mutations that affect single amino acids showed that the mutant proteins were retained in the ER. These data suggest that CLN6 is an ER resident protein, the activity of which, despite this location, must contribute to lysosomal function.

Mutations in classical late infantile neuronal ceroid lipofuscinosis disrupt transport of tripeptidyl-peptidase I to lysosomes

Classical late infantile neuronal ceroid lipofuscinosis is an autosomal recessive disease caused by mutations in the CLN2 gene resulting in functional defects of the gene product tripeptidyl-peptidase I. This disease is associated with a progressive neurodegenerative course beginning at the age of two years with developmental stagnation, finally leading to a complete loss of motor function, vision and speech by the age of 10 years. We analyzed the functional consequences of the mutations R127Q, R208X, N286S, I287N, T353P and Q422H, which were previously identified in patients with late infantile ceroid lipofuscinosis, with regard to enzymatic activity, stability, post-translational processing and intracellular localization of tripeptidyl-peptidase I. We could not detect any translational product for the mutant R208X. We found that four missense mutations, N286S, I287N, T353P and Q422H, which are located in conserved protein regions of tripeptidyl-peptidase I, decreased the enzymatic activity dramatically, blocked processing to mature size peptidase and led to protein retention in the endoplasmatic reticulum and rapid degradation in non-lysosomal compartments. We conclude that these amino-acid substitutions induce major misfolding of the precursor peptidase and hence prevent post-translational processing and lysosomal targeting of tripeptidyl-peptidase I. In contrast, the amino-acid substitution R127Q within a non-conserved protein region did not significantly affect enzymatic activity, stability, processing and lysosomal targeting of tripetidyl-peptidase I. Thus, our functional analyses of CLN2 mutations reveal novel insight into the molecular defect underlying dysfunction of tripeptidyl-peptidase I.

The lysosomal degradation of neuromedin B is dependent on tripeptidyl peptidase-I: evidence for the impairment of neuropeptide degradation in late-infantile neuronal ceroid lipofuscinosis

Late-infantile neuronal ceroid lipofuscinosis (CLN2), previously known as the late-infantile form of Batten disease, is a lysosomal storage disease which results from mutations in the gene that codes for tripeptidyl peptidase-I (TPP-I). This disease is characterised by progressive neurodegeneration in young children although the molecular mechanisms responsible for neuronal cell death are unclear. TPP-I is an exopeptidase which removes N-terminal tripeptides from small peptides, including several peptide hormones. We report that the degradation of the neuropeptide, neuromedin B, by mouse brain cells is restricted to lysosomes and that the pattern of degradation products is consistent with a predominant role for TPP-I. Neuromedin B is degraded by a similar pathway in a mouse neuronal cell line and also in cultured human fibroblasts. A specific inhibitor of TPP-I is able to abolish neuromedin B degradation in a variety of cell types. Fibroblasts from CLN2 patients, which are deficient in TPP-I activity, are unable to degrade neuromedin B. These observations suggest that TPP-I is the predominant proteolytic enzyme responsible for the intracellular degradation of neuromedin B. The inability of cells from CLN2 patients to degrade neuromedin B and other neuropeptides may contribute to the pathogenesis of the disease.

Neuronal and microglial cathepsins in aging and age-related diseases

It has been long believed that cathepsins compensate for each other because of their overlapping substrate specificities. However, there is increasing evidence that disturbance of the normal balance of their enzymatic activities is the first insult in brain aging and age-related diseases. The imbalance of cathepsins may further cause age-related neuropathological changes such as accumulation of autophagic vacuoles and the formation of ceroid-lipofuscin leading to neuronal dysfunction and damage. Leakage of cathepsins due to the fragility of lysosomal membranes during aging also contributes to neurodegeneration. Furthermore, the deficiency of cathepsin D has been recently revealed to provoke a novel type of lysosomal storage disease associated with massive neurodegeneration. In these animals, microglia are activated to initiate inflammatory and cytotoxic responses by binding and phagocytosis of storage neurons. Activated microglia also release some members of cathepsins to induce neuronal death by degrading extracellular matrix proteins. Thus the microglial activation possibly through sensing neuronal storage may also be an important causative factor for neurodegeneration in lysosomal storage diseases and age-related diseases such as Alzheimer's disease. This review describes the pathological roles of neuronal and microglial cathepsins in brain aging and age-related diseases.

Selectivity and types of cell death in the neuronal ceroid lipofuscinoses

Cloning of the individual genes that are mutated in the neuronal ceroid lipofuscinoses (NCLs), or Batten disease, has opened up new avenues of research into the pathogenesis of these fatal autosomal recessive storage disorders. Genetically accurate mouse models have now been generated for each major form of the disorder, together with several variant forms. Ongoing analysis of these mice is revealing significant new data about the staging and progression of disease phenotypes. Combined with data from human autopsy tissues and large animal models, it is now clear that neurodegeneration is initially selective in the NCL CNS, targeting specific regions and particular cell populations. There is also evidence of selective glial activation that appears to precede obvious neurodegeneration, becoming more widespread with disease progression. Currently, there is debate over the mechanisms of cell death that operate in each form of NCL, with evidence of both apoptosis and autophagy. It is likely that these mechanisms may encompass a spectrum of cell death events, depending upon the specific context of each neuronal population. Taken together, these data have significant clinical implications for the development and targeting of appropriate therapeutic strategies, and for providing the landmarks to judge their efficacy.

The intracellular location and function of proteins of neuronal ceroid lipofuscinoses

Neuronal ceroid lipofuscinoses are a group of diseases characterized by accumulation of hydrophobic proteins in lysosomes of neurons and other types of cells. NCLs are caused by at least 8 mutant genes (CLN1-CLN8), though CLN4 and CLN7 have not yet been identified. Except for Cln1p, the protein encoded by CLN1, the defective proteins are associated with lysosomal accumulation of mitochondrial ATP synthase subunit c. Cln1p and Cln2p are soluble lysosomal enzymes, targeted to lysosomes in a mannose 6-phosphate dependent manner. Mutations in the lysosomal protease cathepsin D cause another NCL. Cln3p, Cln5p, Cln6p and Cln8p are thought to be transmembrane proteins. Cln3p and Cln5p are localized in the endosome-lysosomal compartment. Deficiency of endosomal membrane protein CLC-3, a member of the chloride channel family, causes NCL-like phenotype and lysosomal storage of subunit c. Herein, we review the features of NCL and NCL-related proteins and discuss the involvement of the proteins in lysosomal degradation of subunit c.

The analytical approach to the nature of lipofuscin (age pigment)

Analytical studies of three lipopigments show that much can be achieved. Lipopigment from ovine ceroid-lipofuscinosis is composed of discrete protein and lipid molecules in orderly arrays and lipid peroxidation is not involved in its formation. Subunit c of mitochondrial ATP synthase accounts for approximately 50% of accumulated material and is specific to the disease process in this and other forms of the disease. Lipofuscin from bovine heart was mostly soluble and also contained discrete proteins, lipids and metals. Equine thyroid lipofuscin was less soluble but also had a relatively high protein content, probably derived from thyroglobulin. Although sugar could not be measured quantitatively, staining reactions and elemental analyses suggested it could also be a significant component. Some may be present as derivatives in the form of advanced glycation products. It is proposed that protein, the dominant molecular species present, is the important constituent in lipofuscinogenesis rather than lipid peroxidation. Whereas this latter may play some part in the maturation of lipofuscin, this has not been shown experimentally and is not likely to be the initiating mechanism.