Structural organization and sequence of CLN2, the defective gene in classical late infantile neuronal ceroid lipofuscinosis

The Role of Visual Electrophysiology in Systemic Hereditary Syndromes

Visual electrophysiology is a valuable tool for evaluating the visual system in various systemic syndromes. This review highlights its clinical application in a selection of syndromes associated with hearing loss, mitochondrial dysfunction, obesity, and other multisystem disorders. Techniques such as full-field electroretinography (ffERG), multifocal electroretinography (mfERG), pattern electroretinography (PERG), visual evoked potentials (VEP), and electrooculography (EOG) offer insights into retinal and optic nerve function, often detecting abnormalities before clinical symptoms manifest. In hearing loss syndromes like Refsum disease, Usher syndrome (USH), and Wolfram syndrome (WS), electrophysiology facilitates the detection of early retinal changes that precede the onset of visual symptoms. For mitochondrial disorders such as maternally-inherited diabetes and deafness (MIDD), Kearns-Sayre syndrome (KSS), and neuropathy, ataxia, and retinitis pigmentosa (NARP) syndrome, these tests can be useful in characterizing retinal degeneration and optic neuropathy. In obesity syndromes, including Bardet-Biedl syndrome (BBS), Alström syndrome, and Cohen syndrome, progressive retinal degeneration is a hallmark feature. Electrophysiological techniques aid in pinpointing retinal dysfunction and tracking disease progression. Other syndromes, such as Alagille syndrome (AGS), abetalipoproteinemia (ABL), Cockayne syndrome (CS), Joubert syndrome (JS), mucopolysaccharidosis (MPS), Neuronal ceroid lipofuscinoses (NCLs), and Senior-Løken syndrome (SLS), exhibit significant ocular involvement that can be evaluated using these methods. This review underscores the role of visual electrophysiology in diagnosing and monitoring visual system abnormalities across a range of syndromes, potentially offering valuable insights for early diagnosis, monitoring of progression, and management.

Genetic spectrum of neuronal ceroid lipofuscinosis & its genotype-phenotype correlation -A single centre experience of 56 cases

BACKGROUND

Neuronal ceroid lipofuscinoses (NCLs) are progressive, autosomal recessive lysosomal storage disorders primarily affecting children, marked by seizures, cognitive decline, motor regression, and visual impairment. Limited genetic data exist for South Asian populations, with most studies relying on enzymatic assays or electron microscopy. This study explores the genetic spectrum of NCL and genotype-phenotype correlations in a cohort from South India.

METHODS

A retrospective analysis was conducted on 56 genetically confirmed NCL patients diagnosed between January 2018 and June 2024 at a specialized neurological center in South India. Genetic analysis using next-generation sequencing (NGS) were performed, with variants classified as per ACMG guidelines. Clinical, electroencephalographic (EEG), imaging, and electron microscopy (EM) findings were reviewed, and genotype-phenotype correlations were analyzed.

RESULTS

The cohort (33 males, 23 females) had a median age of onset of 36 months and a median disease duration of 65.5 months. Eight genetic subtypes were identified, with predominant mutations in TPP1 (19.64%), CLN6, MFSD8, and CLN8 (16.07% each). Seizures (75%), regression of milestones (87.5%), visual impairment (33.9%), and ataxia (57.1%) were common. EEG abnormalities were found in 76.3%, MRI revealed cerebellar atrophy in 89.13%, and thalamic T2 hypo-intensity in 91.3%. EM showed curvilinear and fingerprint profiles. Of the identified variants, 31 were previously reported, while 29 were novel.

CONCLUSION

This is the largest single-center NCL cohort in South Asia, highlighting a diverse genetic spectrum and significant novel variants, underscoring the importance of genetic testing for diagnosis and future therapies.

An ERG and OCT study of neuronal ceroid lipofuscinosis CLN2 Battens retinopathy

BACKGROUND

Late infantile neuronal ceroid lipofuscinosis (CLN2 Batten disease) is a rare, progressive neurodegenerative disease of childhood. The natural history of motor and language regression is used to monitor the efficacy of CNS treatments. Less is known about CLN2 retinopathy. Our aim is to elaborate the nature, age of onset, and symmetry of CLN2 retinopathy using visual electrophysiology and ophthalmic imaging.

SUBJECTS AND METHODS

We reviewed 22 patients with genetically confirmed CLN2 disease; seventeen showing classical and five atypical disease. Flash electroretinograms (ERGs), flash and pattern reversal visual evoked potentials (VEPs), recorded from awake children were collated. Available fundus images were graded, optical coherence tomography (OCT) central subfoveal thickness (CST) measured, and genotype, age, clinical vision assessment and motor language grades assembled.

RESULTS

ERGs show cone/rod system dysfunction preceded by localised macular ellipsoid zone disruption on OCT from 4.8 years. Electroencephalogram (EEG) time-locked spikes confounded both pattern 6/17 (35%) and flash VEPs 12/16 (75%). Paired right eye (RE) and left eye (LE) ERG amplitudes did not differ significantly for each flash stimulus at the p 0.001 level, Wilcoxon ranked signed test. Cone ERGs show a functional deficit before CST thinning in classical disease. Optomap hyper fundus autofluorescence (FAF) at the fovea was noted in three patients with normal ERGs. The oldest patient showed an ovoid aggregate above the external limiting membrane at the fovea, which did not affect the PERG.

CONCLUSION

ERG findings in CLN2 retinopathy show symmetrical cone-rod dysfunction, from 4y10m in this series, but a broad range of ages when ERG function is preserved.

Generation of pathogenic TPP1 mutations in human stem cells as a model for neuronal ceroid lipofuscinosis type 2 disease

Neuronal ceroid lipofuscinosis type 2 (CLN2 disease) is an autosomal recessive neurodegenerative disorder generally with onset at 2 to 4 years of age and characterized by seizures, loss of vision, progressive motor and mental decline, and premature death. CLN2 disease is caused by loss-of-function mutations in the tripeptidyl peptidase 1 (TPP1) gene leading to deficiency in TPP1 enzyme activity. Approximately 60% of patients have one of two pathogenic variants (c.509–1G > C or c.622C > T [p.(Arg208*)]). In order to generate a human stem cell model of CLN2 disease, we used CRISPR/Cas9-mediated knock-in technology to introduce these mutations in a homozygous state into H9 human embryonic stem cells. Heterozygous lines of the c.622C > T (p.(Arg208*)) mutation were also generated, which included a heterozygous mutant with a wild-type allele and different compound heterozygous coding mutants resulting from indels on one allele. We describe the methodology that led to the generation of the lines and provide data on the initial validation and characterization of these CLN2 disease models. Notably, both mutant lines (c.509–1G > C and c.622C > T [p.(Arg208*)]) in the homozygous state were shown to have reduced or absent protein, respectively, and deficiency of TPP1 enzyme activity. These models, which we have made available for wide-spread sharing, will be useful for future studies of molecular and cellular mechanisms underlying CLN2 disease and for therapeutic development.

Intravitreal enzyme replacement preserves retinal structure and function in canine CLN2 neuronal ceroid lipofuscinosis

CLN2 neuronal ceroid lipofuscinosis is a hereditary neurodegenerative disorder characterized by progressive vision loss, neurological decline, and seizures. CLN2 disease results from mutations in TPP1 that encodes the lysosomal enzyme tripeptidyl peptidase-1 (TPP1). Children with CLN2 neuronal ceroid lipofuscinosis experience ocular disease, characterized by progressive retinal degeneration associated with impaired retinal function and gradual vision loss culminating in total blindness. A similar progressive loss of retinal function is also observed in a dog CLN2 model with a TPP1 null mutation. A study was conducted to evaluate the efficacy of periodic intravitreal injections of recombinant human (rh) TPP1 in inhibiting retinal degeneration and preserving retinal function in the canine model. TPP1 null dogs received periodic intravitreal injections of rhTPP1 in one eye and vehicle in the other eye beginning at approximately 12 weeks of age. Ophthalmic exams, in vivo ocular imaging, and electroretinography (ERG) were repeated regularly to monitor retinal structure and function. Retinal histology was evaluated in eyes collected from these dogs when they were euthanized at end-stage neurological disease (43-46 weeks of age). Intravitreal rhTPP1 dosing prevented disease-related declines in ERG amplitudes in the TPP1-treated eyes. At end-stage neurologic disease, TPP1-treated eyes retained normal morphology while the contralateral vehicle-treated eyes exhibited loss of inner retinal neurons and photoreceptor disorganization typical of CLN2 disease. The treatment also prevented the development of disease-related focal retinal detachments observed in the control eyes. Uveitis occurred secondary to the administration of the rhTPP1 but did not hinder the therapeutic benefits. These findings demonstrate that periodic intravitreal injection of rhTPP1 preserves retinal structure and function in canine CLN2 disease.

Drugs modulating stochastic gene expression affect the erythroid differentiation process

To better understand the mechanisms behind cells decision-making to differentiate, we assessed the influence of stochastic gene expression (SGE) modulation on the erythroid differentiation process. It has been suggested that stochastic gene expression has a role in cell fate decision-making which is revealed by single-cell analyses but studies dedicated to demonstrate the consistency of this link are still lacking. Recent observations showed that SGE significantly increased during differentiation and a few showed that an increase of the level of SGE is accompanied by an increase in the differentiation process. However, a consistent relation in both increasing and decreasing directions has never been shown in the same cellular system. Such demonstration would require to be able to experimentally manipulate simultaneously the level of SGE and cell differentiation in order to observe if cell behavior matches with the current theory. We identified three drugs that modulate SGE in primary erythroid progenitor cells. Both Artemisinin and Indomethacin decreased SGE and reduced the amount of differentiated cells. On the contrary, a third component called MB-3 simultaneously increased the level of SGE and the amount of differentiated cells. We then used a dynamical modelling approach which confirmed that differentiation rates were indeed affected by the drug treatment. Using single-cell analysis and modeling tools, we provide experimental evidence that, in a physiologically relevant cellular system, SGE is linked to differentiation.

Upregulation of tripeptidyl-peptidase 1 by 3-hydroxy-(2,2)-dimethyl butyrate, a brain endogenous ligand of PPARα: Implications for late-infantile Batten disease therapy

The late-infantile Batten disease or late-infantile neuronal ceroid lipofuscinosis (LINCL) is an autosomal recessive lysosomal storage disorder caused by mutations in the Cln2 gene leading to deficiency of lysosomal enzyme tripeptidyl peptidase 1 (TPP1). At present, available options for this fatal disorder are enzyme replacement therapy and gene therapy, which are extensively invasive and expensive. Our study demonstrates that 3-hydroxy-(2,2)-dimethyl butyrate (HDMB), a brain endogenous molecule, is capable of stimulating TPP1 expression and activity in mouse primary astrocytes and a neuronal cell line. HDMB activated peroxisome proliferator-activated receptor-α (PPARα), which, by forming heterodimer with Retinoid X receptor-α (RXRα), transcriptionally upregulated the Cln2 gene. Moreover, by using primary astrocytes from wild type, PPARα-/- and PPARβ-/- mice, we demonstrated that HDMB specifically required PPARα for inducing TPP1 expression. Finally, oral administration of HDMB to Cln2 heterozygous (Cln2+/-) mice led to a marked upregulation of TPP1 expression in the motor cortex and striatum in a PPARα-dependent fashion. Our study suggests that HDMB, a brain endogenous ligand of PPARα, might have therapeutic importance for LINCL treatment.

Proteomic Analysis of Brain and Cerebrospinal Fluid from the Three Major Forms of Neuronal Ceroid Lipofuscinosis Reveals Potential Biomarkers

Clinical trials have been conducted for the neuronal ceroid lipofuscinoses (NCLs), a group of neurodegenerative lysosomal diseases that primarily affect children. Whereas clinical rating systems will evaluate long-term efficacy, biomarkers to measure short-term response to treatment would be extremely valuable. To identify candidate biomarkers, we analyzed autopsy brain and matching CSF samples from controls and three genetically distinct NCLs due to deficiencies in palmitoyl protein thioesterase 1 (CLN1 disease), tripeptidyl peptidase 1 (CLN2 disease), and CLN3 protein (CLN3 disease). Proteomic and biochemical methods were used to analyze lysosomal proteins, and, in general, we find that changes in protein expression compared with control were most similar between CLN2 disease and CLN3 disease. This is consistent with previous observations of biochemical similarities between these diseases. We also conducted unbiased proteomic analyses of CSF and brain using isobaric labeling/quantitative mass spectrometry. Significant alterations in protein expression were identified in each NCL, including reduced STXBP1 in CLN1 disease brain. Given the confounding variable of post-mortem changes, additional validation is required, but this study provides a useful starting set of candidate NCL biomarkers for further evaluation.

The paediatric rheumatologist and orphan disease - a story without happy ending

Orphan diseases are not a common challenge in the everyday practice of the rheumatologist. Despite their extremely rare occurrence one of the patients under our care developed one of them – neuronal ceroid lipofuscinosis, the most frequent neurodegenerative disease observed in the paediatric population. We report a case of 2-year-old girl diagnosed with oligoarticular form of juvenile idiopathic arthritis treated in our Department with steroids and methotrexate and staying in the stage of disease remission. During routine checkups at Outpatient Clinic we observed progressive deterioration of girls neurological condition resulting in ataxia, gait disturbances with no rheumatological cause behind and speech impairment. The appearance of the symptoms was accompanied by frequent episodes of epileptic seizures, with little clinical improvement on combined antiepileptic treatment. Magnetic resonance imaging that we performed showed a picture highly suggestive of neuronal ceroid lipofuscinosis – atrophy of the patients cerebrum and cerebellum. Genetic testing conducted resulted in the diagnosis of late infantile neuronal ceroid lipofuscinosis (LINCL).

Neuronal ceroid lipofuscinosis type CLN2: a new rationale for the construction of phenotypic subgroups based on a survey of 25 cases in South America

Tripeptidyl-peptidase 1 (TPP1) null or residual activity occurs in neuronal ceroid lipofuscinosis (NCL) with underlying TPP1/CLN2 mutations. A survey of 25 South American CLN2 affected individuals enabled the differentiation of two phenotypes: classical late-infantile and variant juvenile, each in approximately 50% of patients, with residual TPP1 activity occurring in approximately 32%. Each individual was assigned to one of three subgroups: (I) n=11, null TPP1 activity in leukocytes; (II) n=8, residual TPP1 activity of 0.60-15.85 nmol/h/mg (nr 110-476); (III) n=6, activity not measured in leukocytes. Curvilinear bodies (CB) appeared in almost all studied CLN2 subjects; the only exceptions occurred in cases of subgroup II: two individuals had combined CBs/fingerprints (FPs), and one case had pure FPs. There were 15 mutations (4 first published in this paper, 3 previously observed in South America by our group, and 8 previously observed by others). In subgroup I, mutations were either missense or nonsense; in subgroups II and III, mutations prevailed at the non-conserved intronic site, c.887-10A>G (intron 7), and to a lesser extent at c.89+5G>C (intron 2), in heterozygous combinations. Grouping phenotypically and genetically known individuals on the basis of TPP1 activity supported the concept that residual enzyme activity underlies a protracted disease course. The prevalence of intronic mutations at non-conserved sites in subgroup II individuals indicates that some alternative splicing might allow some residual TPP1 activity.

Analysis of NCL Proteins from an Evolutionary Standpoint

The Neuronal Ceroid Lipofuscinoses (NCLs) are the most common group of neurodegenerative disorders of childhood. While mutations in eight different genes have been shown to be responsible for these clinically distinct types of NCL, the NCLs share many clinical and pathological similarities. We have conducted an exhaustive Basic Local Alignment Search Tool (BLAST) analysis of the human protein sequences for each of the eight known NCL proteins- CLN1, CLN2, CLN3, CLN5, CLN6, CLN7, CLN8 and CLN10. The number of homologous species per CLN-protein identified by BLAST searches varies depending on the parameters set for the BLAST search. For example, a lower threshold is able to pull up more homologous sequences whereas a higher threshold decreases this number. Nevertheless, the clade confines are consistent despite this variation in BLAST searching parameters. Further phylogenetic analyses on the appearance of NCL proteins through evolution reveals a different time line for the appearance of the CLN-proteins. Moreover, divergence of each protein shows a different pattern, providing important clues on the evolving role of these proteins. We present and review in-depth bioinformatic analysis of the NCL proteins and classify the CLN-proteins into families based on their structures and evolutionary relationships, respectively. Based on these analyses, we have grouped the CLN-proteins into common clades indicating a common evolving pathway within the evolutionary tree of life. CLN2 is grouped in Eubacteria, CLN1 and CLN10 in Viridiplantae, CLN3 in Fungi/ Metazoa, CLN7 in Bilateria and CLN5, CLN6 and CLN8 in Euteleostomi.

A critical tryptophan and Ca2+ in activation and catalysis of TPPI, the enzyme deficient in classic late-infantile neuronal ceroid lipofuscinosis

Background

Tripeptidyl aminopeptidase I (TPPI) is a crucial lysosomal enzyme that is deficient in the fatal neurodegenerative disorder called classic late-infantile neuronal ceroid lipofuscinosis (LINCL). It is involved in the catabolism of proteins in the lysosomes. Recent X-ray crystallographic studies have provided insights into the structural/functional aspects of TPPI catalysis, and indicated presence of an octahedrally coordinated Ca²⁺.

Methodology

Purified precursor and mature TPPI were used to study inhibition by NBS and EDTA using biochemical and immunological approaches. Site-directed mutagenesis with confocal imaging technique identified a critical W residue in TPPI activity, and the processing of precursor into mature enzyme.

Principal Findings

NBS is a potent inhibitor of the purified TPPI. In mammalian TPPI, W542 is critical for tripeptidyl peptidase activity as well as autocatalysis. Transfection studies have indicated that mutants of the TPPI that harbor residues other than W at position 542 have delayed processing, and are retained in the ER rather than transported to lysosomes. EDTA inhibits the autocatalytic processing of the precursor TPPI.

Conclusions/Significance

We propose that W542 and Ca²⁺ are critical for maintaining the proper tertiary structure of the precursor proprotein as well as the mature TPPI. Additionally, Ca²⁺ is necessary for the autocatalytic processing of the precursor protein into the mature TPPI. We have identified NBS as a potent TPPI inhibitor, which led in delineating a critical role for W542 residue. Studies with such compounds will prove valuable in identifying the critical residues in the TPPI catalysis and its structure-function analysis.

Lysosomal serine protease CLN2 regulates tumor necrosis factor-alpha-mediated apoptosis in a Bid-dependent manner

Apoptosis is a highly organized, energy-dependent program by which multicellular organisms eliminate damaged, superfluous, and potentially harmful cells. Although caspases are the most prominent group of proteases involved in the apoptotic process, the role of lysosomes has only recently been unmasked. This study investigated the role of the lysosomal serine protease CLN2 in apoptosis. We report that cells isolated from patients affected with late infantile neuronal ceroid lipofuscinosis (LINCL) having a deficient activity of CLN2 are resistant to the toxic effect of death ligands such as tumor necrosis factor (TNF), CD95 ligand, or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) but not to receptor-independent stress agents. CLN2-deficient cells exhibited a defect in TNF-induced Bid cleavage, release of cytochrome c, and caspase-9 and -3 activation. Moreover, extracts from CLN2-overexpressing cells or a CLN2 recombinant protein were able to catalyze the in vitro cleavage of Bid. Noteworthy, correction of the lysosomal enzyme defect of LINCL fibroblasts using a medium enriched in CLN2 protein enabled restoration of TNF-induced Bid and caspase-3 processing and toxicity. Conversely, transfection of CLN2-corrected cells with small interfering RNA targeting Bid abrogated TNF-induced cell death. Altogether, our study demonstrates that genetic deletion of the lysosomal serine protease CLN2 and the subsequent loss of its catalytic function confer resistance to TNF in non-neuronal somatic cells, indicating that CLN2 plays a yet unsuspected role in TNF-induced cell death.

Tripeptidyl-peptidase I in health and disease

The lysosomal lumen contains numerous acidic hydrolases involved in the degradation of carbohydrates, lipids, proteins, and nucleic acids, which are basic cell components that turn over continuously within the cell and/or are ingested from outside of the cell. Deficiency in almost any of these hydrolases causes accumulation of the undigested material in secondary lysosomes, which manifests itself as a form of lysosomal storage disorder (LSD). Mutations in tripeptidyl-peptidase I (TPP I) underlie the classic late-infantile form of neuronal ceroid lipofuscinoses (CLN2), the most common neurodegenerative disorders of childhood. TPP I is an aminopeptidase with minor endopeptidase activity and Ser475 serving as an active-site nucleophile. The enzyme is synthesized as a highly glycosylated precursor transported by mannose-6-phosphate receptors to lysosomes, where it undergoes proteolytic maturation. This review summarizes recent progress in understanding of TPP I biology and molecular pathology of the CLN2 disease process, including distribution of the enzyme, its biosynthesis, glycosylation, transport and activation, as well as catalytic mechanisms and their potential implications for pathogenesis and treatment of the underlying disease. Promising data from gene and stem cell therapy in laboratory animals raise hope that CLN2 will be the first neurodegenerative LSD for which causative treatment will become available for humans.

Clinical protocol. Administration of a replication-deficient adeno-associated virus gene transfer vector expressing the human CLN2 cDNA to the brain of children with late infantile neuronal ceroid lipofuscinosis

Late infantile neuronal ceroid lipofuscinosis (LINCL) is a fatal childhood neurodegenerative lysosomal storage disease with no known therapy. There are estimated to be 200 to 300 children in the United States at any one time with the disease. LINCL is a genetic disease resulting from a deficiency of tripeptidyl peptidase I (TPP-I), a proteolytic enzyme encoded by CLN2, the gene that is mutated in individuals with LINCL. The subjects are chronically ill, with a progressive CNS disorder that invariably results in death, typically by age 8 to 12 years. The strategy of this clinical study is based on the concept that persistent expression in the CNS of the normal CLN2 cDNA with production of sufficient amounts of TPP-I should prevent further loss of neurons, and hence limit disease progression. To assess this concept, an adeno-associated virus vector (AAV2CUh-CLN2) will be used to transfer to and express the human CLN2 cDNA in the brain of children with LINCL. The vector consists of the AAV2 capsid enclosing the 4278-base single-stranded genome consisting of the two inverted terminal repeats of AAV serotype 2 and an expression cassette composed of the human cytomegalovirus (CMV) enhancer, the chicken beta-actin promoter/splice donor and 5' end of the intron, the 3' end of the rabbit P-globin intron and splice acceptor, the human CLN2 cDNA with an optimized Kozak translation initiation signal, and the polyadenylation/transcription stop codon from rabbit 3-globin. The proposed study will include 10 individuals and will be divided into two parts. Group A, to be studied first, will include four individuals with the severe form of the disease. Group B of the trial will include six individuals with a moderate form of the disease. After direct intracranial administration of the vector, there will be neurological assessment based on the LINCL clinical rating scale and magnetic resonance imaging/magnetic resonance spectroscopy assessment of the brain in regions of vector administration. The data generated will help evaluate two hypotheses: (1) that it is safe to carry out direct intracranial administration of the AAV2cuhCLN2 vector to the CNS of individuals with LINCL, and (2) that administration of the AAV2cuhCLN2 vector will slow down or halt the progression of the disease in the central nervous system.

Glycosaminoglycans Modulate Activation, Activity, and Stability of Tripeptidyl-peptidase I in Vitro and in Vivo

Tripeptidyl-peptidase I (TPP I, CLN2 protein) is a lysosomal exopeptidase that sequentially removes tripeptides from the N termini of polypeptides and shows a minor endoprotease activity. Mutations in TPP I lead to classic late-infantile neuronal ceroid lipofuscinosis, a neurodegenerative lysosomal storage disease. TPP I proenzyme is converted in lysosomes into a mature enzyme with the assistance of another protease and is able to autoactivate in acidic pH in vitro via a unimolecular mechanism. Because autoactivation in vitro at the pH values reported for lysosomes generated inactive enzyme, we intended to determine whether physiologically relevant factors can modify this process to also make it plausible in vivo. Here, we report that high ionic strength and glycosaminoglycans (GAGs) increase yields (ionic strength) or yields and rates (GAGs) of activation, enhance degradation of liberated TPP I prosegment fragments, and switch effective autoactivation of TPP I proenzyme toward less acidic pH values (up to pH 6.0). Although ionic strength and GAGs also inhibited TPP I activity in vitro and in living cells, the degree of inhibition (from 20 to 60%) appears to be of rather limited functional significance. Importantly, binding to GAGs improved thermal stability of TPP I and protected the enzyme against alkaline pH-induced denaturation in vitro (t((1/2)) of mature enzyme at pH 7.4 increased by approximately 8-fold in the presence of heparin) and in vivo ( approximately 2-fold higher loss of TPP I in cells deficient in GAGs than in control cells after bafilomycin A1 treatment). These findings elucidate a potent physiologically relevant mechanism of TPP I regulation by GAGs and suggest that generation of the active enzyme via autoactivation can be accomplished not only in vitro but in vivo as well.

Ser475, Glu272, Asp276, Asp327, and Asp360 are involved in catalytic activity of human tripeptidyl-peptidase I

Tripeptidyl-peptidase I (TPP I) is a lysosomal aminopeptidase that sequentially removes tripeptides from small polypeptides and also shows a minor endoprotease activity. Mutations in TPP I are associated with a fatal lysosomal storage disorder--the classic late-infantile form of neuronal ceroid lipofuscinoses. In the present study, we analyzed the catalytic mechanism of the human enzyme by using a site-directed mutagenesis. We demonstrate that apart from previously identified Ser475 and Asp360, also Glu272, Asp276, and Asp327 are important for catalytic activity of the enzyme. Involvement of serine, glutamic acid, and aspartic acid in the catalytic reaction validates the idea, formulated on the basis of significant amino acid sequence homology and inhibition studies, that TPP I is the first mammalian representative of a growing family of serine-carboxyl peptidases.

A mouse model of classical late-infantile neuronal ceroid lipofuscinosis based on targeted disruption of the CLN2 gene results in a loss of tripeptidyl-peptidase I activity and progressive neurodegeneration

Mutations in the CLN2 gene, which encodes a lysosomal serine protease, tripeptidyl-peptidase I (TPP I), result in an autosomal recessive neurodegenerative disease of children, classical late-infantile neuronal ceroid lipofuscinosis (cLINCL). cLINCL is inevitably fatal, and there currently exists no cure or effective treatment. In this report, we provide the characterization of the first CLN2-targeted mouse model for cLINCL. CLN2-targeted mice were fertile and apparently healthy at birth despite an absence of detectable TPP I activity. At approximately 7 weeks of age, neurological deficiencies became evident with the onset of a tremor that became progressively more severe and was eventually accompanied by ataxia. Lifespan of the affected mice was greatly reduced (median survival, 138 d), and extensive neuronal pathology was observed including a prominent accumulation of cytoplasmic storage material within the lysosomal-endosomal compartment, a loss of cerebellar Purkinje cells, and widespread axonal degeneration. The CLN2-targeted mouse therefore recapitulates much of the pathology and clinical features of cLINCL and represents an animal model that should provide clues to the normal cellular function of TPP I and the pathogenic processes that underlie neuronal death in its absence. In addition, the CLN2-targeted mouse also represents a valuable model for the evaluation of different therapeutic strategies.

Maturation of Human Tripeptidyl-peptidase I in Vitro

Tripeptidyl-peptidase I (TPP I, CLN2 protein) is a lysosomal aminopeptidase that cleaves off tripeptides from the free N termini of oligopeptides and also shows minor endopeptidase activity. TPP I is synthesized as a preproenzyme. Its proenzyme autoactivates under acidic conditions in vitro, resulting in a rapid conversion into the mature form. In this study, we examined the process of maturation in vitro of recombinant latent human TPP I purified to homogeneity from secretions of Chinese hamster ovary cells overexpressing TPP I cDNA. Autoprocessing of TPP I proenzyme was carried out at a wide pH range, from approximately 2.0 to 6.0, albeit with different efficiencies depending on the pH and the type of buffer. However, the acquisition of enzymatic activity in the same buffer took place in a narrower pH "window," usually in the range of 3.6-4.2. N-terminal sequencing revealed that mature, inactive enzyme generated during autoactivation at higher pH contained N-terminal extensions (starting at 6 and 14 amino acid residues upstream of the prosegment/mature enzyme junction), which could contribute to the lack of activity of TPP I generated in this manner. Autoprocessing was not associated with any major changes of the secondary structure of the proenzyme, as revealed by CD spectroscopy. Both the activation and proteolytic processing of the recombinant TPP I precursor were primarily concentration-independent. The addition of the mature enzyme did not accelerate the processing of the proenzyme. In addition, the maturation of the proenzyme was not affected by the presence of glycerol. Finally, the proenzyme with the active site mutated (S475L) was not processed in the presence of the wild-type enzyme. All of these findings indicate a primarily intramolecular (unimolecular) mechanism of TPP I activation and autoprocessing and suggest that in vivo mature enzyme does not significantly participate in its own generation from the precursor.

N-glycosylation is crucial for folding, trafficking, and stability of human tripeptidyl-peptidase I

Tripeptidyl-peptidase I (TPP I) is a lysosomal serine-carboxyl peptidase that sequentially removes tripeptides from polypeptides. Naturally occurring mutations in TPP I are associated with the classic late infantile neuronal ceroid lipofuscinosis. Human TPP I has five potential N-glycosylation sites at Asn residues 210, 222, 286, 313, and 443. To analyze the role of N-glycosylation in the function of the enzyme, we obliterated each N- glycosylation consensus sequence by substituting Gln for Asn, either individually or in combinations, and expressed mutated cDNAs in Chinese hamster ovary and human embryonic kidney 293 cells. Here, we demonstrate that human TPP I in vivo utilizes all five N-glycosylation sites. Elimination of one of these sites, at Asn-286, dramatically affected the folding of the enzyme. However, in contrast to other misfolded proteins that are retained in the endoplasmic reticulum, only a fraction of misfolded TPP I mutant expressed in Chinese hamster ovary cells, but not in human embryonic kidney 293 cells, was arrested in the ER, whereas its major portion was secreted. Secreted proenzyme formed non-native, interchain disulfide bridges and displayed only residual TPP I activity upon acidification. A small portion of TPP I missing Asn-286-linked glycan reached the lysosome and was processed to an active species; however, it showed low thermal and pH stability. N-Glycans at Asn-210, Asn-222, Asn-313, and Asn-443 contributed slightly to the specific activity of the enzyme and its resistance to alkaline pH-induced inactivation. Phospholabeling experiments revealed that N-glycans at Asn-210 and Asn-286 of TPP I preferentially accept a phosphomannose marker. Thus, a dual role of oligosaccharide at Asn-286 in folding and lysosomal targeting could contribute to the unusual, but cell type-dependent, fate of misfolded TPP I conformer and represent the molecular basis of the disease process in subjects with naturally occurring missense mutation at Asn-286.

Biosynthesis, glycosylation, and enzymatic processing in vivo of human tripeptidyl-peptidase I

Human tripeptidyl-peptidase I (TPP I, CLN2 protein) is a lysosomal serine protease that removes tripeptides from the free N termini of small polypeptides and also shows a minor endoprotease activity. Due to various naturally occurring mutations, an inherited deficiency of TPP I activity causes a fatal lysosomal storage disorder, classic late infantile neuronal ceroid lipofuscinosis (CLN2). In the present study, we analyzed biosynthesis, glycosylation, transport, and proteolytic processing of this enzyme in stably transfected Chinese hamster ovary cells as well as maturation of the endocytosed proenzyme in CLN2 lymphoblasts, fibroblasts, and N2a cells. Human TPP I was initially identified as a single precursor polypeptide of approximately 68 kDa, which, within a few hours, was converted to the mature enzyme of approximately 48 kDa. Compounds affecting the pH of intracellular acidic compartments, those interfering with the intracellular vesicular transport as well as inhibition of the fusion between late endosomes and lysosomes by temperature block or 3-methyladenine, hampered the conversion of TPP I proenzyme into the mature form, suggesting that this process takes place in lysosomal compartments. Digestion of immunoprecipitated TPP I proenzyme with both N-glycosidase F and endoglycosidase H as well as treatment of the cells with tunicamycin reduced the molecular mass of TPP I proenzyme by approximately 10 kDa, which indicates that all five potential N-glycosylation sites in TPP I are utilized. Mature TPP I was found to be partially resistant to endo H treatment; thus, some of its N-linked oligosaccharides are of the complex/hybrid type. Analysis of the effect of various classes of protease inhibitors and mutation of the active site Ser(475) on human TPP I maturation in cultured cells demonstrated that although TPP I zymogen is capable of autoactivation in vitro, a serine protease that is sensitive to AEBSF participates in processing of the proenzyme to the mature, active form in vivo.

Late infantile neuronal ceroid lipofuscinosis: quantitative description of the clinical course in patients with CLN2 mutations

We examined 26 individuals with clinical and electron microscopic signs of late infantile neuronal ceroid lipofuscinosis (LINCL). In 22 cases, we found both pathogenic alleles. Sixteen patients exclusively carried either one or a combination of the two common mutations R208X and IVS5-1G > C. In the remaining cases, four missense mutations could be detected, of which R127Q, N286S, and T353P represent novel, previously not described alleles. A clinical performance score was developed by rating motor, visual, and verbal functions and the incidence of cerebral seizures in 3-month intervals during the course of the disease. A Total Disability Score was derived by summing up the single scores for motor, visual, and verbal functions. The 16 individuals with the two common mutations were grouped together (referred to as standard patients), and the 5th, 50th, and 95th centiles were calculated and graphically depicted over time. The scores for motor function and language ability dropped earliest and progressed very similarly in the standard patients. The performance curves of two children with the N286S mutation slightly diverged from the 95th centile. However, the performance curves of one patient with atypical LINCL carrying the R127Q mutation fell far beyond the 95th centile. The presented performance rating clearly and quantitatively delineates the disease course of the LINCL patients and hence offers a useful tool for clinical evaluation of future therapeutic interventions. In addition, the described performance score system can be applied to other types of neuronal ceroid lipofuscinoses and could be adapted to various other neurodegenerative diseases of childhood.

The expression of tripeptidyl peptidase I in various tissues of rats and mice

To understand the precise distribution of tripeptidyl peptidase I (TPP-I), a defect of which has been shown to induce late infantile neuronal ceroid lipofuscinosis, various tissues from rats and mice were analyzed using biochemical and immunohistochemical techniques. Western blot analyses showed that a protein band immunoreactive to anti-TPP-I appeared in tissue extracts of both animals at a molecular weight of approximately 47 kD. Protein levels of TPP-I differed among tissues; they were high in the rat brain, liver, stomach, kidney, thyroid and adrenal glands and in the mouse brain, stomach, kidney, and testis. The proteolytic activity of TPP-I was detectable; it differed in the tissues examined and did not always reflect the expression levels of the protein in the tissues. In particular, the TPP-I activity was low in the brains of both animals and high in the rat testis, although its protein levels were high in the former tissue and low in the latter. Double immunostaining showed the immunoreactivity for TPP-I to be well localized in granular structures of epithelial cells in renal tubules and the cerebral choroid plexus, both of which were also stained with lamp2, a lysosomal membrane protein marker, indicating that TPP-I is a lysosomal enzyme. The immunoreactivity was intense in F4/80-immunopositive macrophages/microglial cells located in various tissues including the thymus, spleen, liver, alimentary tract, and central nervous system. Although the immunoreactivity differed depending on the tissues and even within the same tissues between the species, it was detected in all tissues examined, especially in nerve cells, some types of endocrine cells, and oxyntic cells such as gastric parietal cells and bone osteoclasts. However, the immunoreactivity was faint and week in rat thyroid gland, although its protein level was high in the tissue. These lines of evidence suggest that TPP-I, a lysosomal serine proteinase, is widely distributed in rat and mouse tissues, although its expression levels vary among them.

The neuronal ceroid lipofuscinoses: mutations in different proteins result in similar disease

The neuronal ceroid-lipofuscinoses (NCL) are the most common group of progressive neurodegenerative diseases in children, with an incidence as high as one in 12,500 live births. The main features of this disease are failure of psychomotor development, impaired vision, seizures, and premature death. Many biochemical and physiological studies have been initiated to determine the cellular defect underlying the disease, although only a few traits have been truly associated with the disorders. One of the paradox's of the NCL-diseases is the characteristic accumulation of autofluorescent hydrophobic material in the lysosomes of neurons and other cell types. However, the accumulation of this lysosomal storage material, which no doubt contributes to the neurologic disease, does not apparently lead to disease outside the CNS, and how these cellular alterations relate to the neurodegeneration in NCLs is unknown. Mutations have been identified in six distinct genes/proteins, namely CLN1, which encodes PPT1, a protein thiolesterase; CLN2, which encodes TPP1, a serine protease; and CLN3, CLN5, CLN6, and CLN8, which encode novel transmembrane proteins. Mutation in any one of these CLN-proteins results in a distinct type of NCL-disease. However, there are many shared similarities in the pathology of these diseases. The most obvious connection between PPT1, TPP1, CLN3, CLN5, CLN6, and CLN8 is their subcellular localization. To date, three of the four proteins whose subcellular localization has been confirmed, namely PPT1, TPP1, and CLN3, reside in the lysosome. We review the function of the CLN-proteins and discuss the possibility that a disruption in a common biological process leads to an NCL-disease.

Rat tripeptidyl peptidase I: molecular cloning, functional expression, tissue localization and enzymatic characterization

We purified tripeptidyl peptidase I (TPP I) to homogeneity from a rat kidney lysosomal fraction and determined its physicochemical properties, including its molecular weight, substrate specificity and partial amino acid sequence. The molecular weight of the enzyme was calculated to be 280,000 and 290,000 by non-denaturing PAGE and gel filtration, respectively, and to be 43 000 and 46 000 on SDS-PAGE in the absence and presence of beta-ME, respectively. These findings suggest that the enzyme is composed of six identical subunits. The Km, Vmax, kcat and kcat/Km values of TPP I at optimal pH (pH 4.0) were 680 microM, 3.7 micromol x mg(-1) x min(-1), 33.1 s(-1) and 4.87 x 10(4) s(-1) x M(-1) for Ala-Ala-Phe-MCA, respectively. TPP I was significantly inhibited by PCMBS and HgCl2, and moderately by DFP. These findings also suggest that TPP I is an exotype serine peptidase that is regulated by SH reagent. TPP I released the tripeptide Arg-Val-Tyr from angiotensin III more rapidly than from Ala-Ala-Phe-MCA, and also released Gly-Asn-Leu from neuromedin B with the same velocity as from Ala-Ala-Phe-MCA. Angiotensin III and neuromedin B have recently been found to be good natural substrates for lysosomal TPP I. Furthermore, we determined the rat liver cDNA structure and deduced the amino acid sequence. The cDNA, designated as lambdaRTI-1, is composed of 2485 bp and encodes 563 amino acids in the coding region. By Northern blot analysis, the order for TPP I mRNA expression was kidney > or = liver > heart > brain > lung > spleen >> skeletal muscle and testis. In parallel experiments, the TPP I antigen was detected in various rat tissues by immunohistochemical staining.

Molecular genetic testing for neuronal ceroid lipofuscinoses

Eight different NCL forms have been recognized to be encoded by genes CLN1-8. CLN1,2,3,5,and 8 have been cloned, and at least 85 mutations have been detected. Molecular technology can now be applied to genetic testing for NCLs; testing is now available in clinic diagnostic and research laboratories for CLN genes that have been cloned. Molecular genetic testing makes it possible not only to confirm clinical and pathological diagnoses but also to offer pre-symptom diagnosis and carrier screening for NCL families. In addition, DNA-based mutation analysis may predict prenatal outcome more accurately for pregnant women in NCL families.

Diagnosis of late-infantile neuronal ceroid lipofuscinosis: a new sensitive method to assay lysosomal pepstatin-insensitive proteinase activity in human and animal specimens by capillary electrophoresis

Batten disease, or human late-infantile neuronal ceroid lipofuscinosis (LINCL) is a familiar progressive degenerative disease affecting children, caused by a deficiency of a lysosomal proteinase (tripeptidyl peptidase I, TPP-I) and characterized by the accumulation of autofluorescent storage bodies in the brain and other tissues of the body. Current methodology used to diagnose this disease needs to be improved in order to have less invasive techniques with higher resolution and shorter assay time. In this report, we discuss the potential merits of micellar electrokinetic chromatography as an excellent tool that requires minute samples but offers high resolution and a short running time for monitoring TPP-I activity in human and animal specimens.

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.

Specific substrate for CLN2 protease/tripeptidyl-peptidase I assay

The classic late infantile neuronal ceroid lipofuscinosis (LINCL, CLN2) is a fatal neurodegenerative disorder that results from mutations in a gene encoding a lysosomal proteinase, known as CLN2 protease (CLN2p) or tripeptidyl peptidase I (TPP-I). Three different substrates, fluorescein isothiocyanate-labelled haemoglobin, A-F-F-7-amino-4-methylcoumarin (AAF-AMC) and G-F-F-L-7-amino-4-trifluoromethylcoumarin (GFFL-AFC) have been used for the CLN2p/TPP-I assay with varying degrees of residual activities in patients with LINCL. Further, conclusive identification of carriers are not possible with the first two substrates. An assay for the CLN2p/TPP-I based on the cleavage of amino terminal tripeptide from G-F-F-L-AFC was applied to prenatal and postnatal diagnosis of LINCL patients and heterozygote carriers. In leukocytes, the CLN2p/TPP-I activities in controls and heterozygote carriers were 1995 +/- 154 (n = 15) and 918 +/- 253 (n = 15) nmol/h/mg protein respectively. No CLN2p/TPP-I activity was detectable in all but two patients. These two patients had less than 2% residual activity, and had delayed clinical symptoms for LINCL. This shows that the G-F-F-L-AFC is a highly specific substrate for the CLN2p/TPP-I assay. The fact that with this substrate the enzyme cleaves a peptide bond between the two amino acids may be the reason for the high level of specificity.

Tripeptidyl-peptidase I in neuronal ceroid lipofuscinoses and other lysosomal storage disorders

The classic late infantile form of neuronal ceroid lipofuscinosis (CLN2, cLINCL) is associated with mutations in the gene encoding tripeptidyl-peptidase I (TPP-I), a lysosomal aminopeptidase that cleaves off tripeptides from the free N-termini of oligopeptides. To date over 30 different mutations and 14 polymorphisms associated with CLN2 disease process have been identified. In the present study, we analysed the molecular basis of 15 different mutations of TPP-I by using immunocytochemistry, immunofluorescence, Western blotting, enzymatic assay and subcellular fractionation. In addition, we studied the expression of TPP-I in other lysosomal storage disorders such as CLN1, CLN3, muccopolysaccharidoses and GM1 and GM2 gangliosidoses. Our study shows that TPP-I is absent or appears in very small amounts not only in cLINCL subjects with mutations producing severely truncated protein, but also in individuals with missense point mutations, which correlates with loss of TPP-I activity. Of interest, small amounts of TPP-I were detected in lysosomal fraction from fibroblasts from cLINCL subject with protracted form. This observation suggests that the presence of small amounts of TPP-I in lysosomes is able to delay significantly CLN2 disease process. We also show that TPP-I immunoreactivity is increased in the brain tissue of CLN1 and CLN3 subjects, stronger in glial cells and macrophages than neurons. Less prominent increase of TPP-I staining was found in muccopolysaccharidoses and GM1 and GM2 gangliosidoses. These data suggest that TPP-I participates in lysosomal turnover of proteins in pathological conditions associated with cell/tissue injury.

Morphological studies on CLN2

Electron microscopic, fluorescence microscopic, and immunohistochemical studies earlier performed on archival cerebral tissue from Max Bielchowsky's original three patients revealed curvilinear bodies rich in subunit C of mitochondrial ATP synthase (SCMAS). Recent progress in the elucidation of CLN2, i.e. identification of the defective lysosomal enzyme tripeptidyl-peptidase I (TPP-I) and mutations in the CLN2 gene have further corroborated earlier data. Immunohistochemically the absence of the TPP-I protein could be confirmed in the archival tissues using pathological controls. Unlike biochemistry, immunohistochemistry enables examination of these archival tissues elucidating the causative defect. Complementary molecular studies identified mutations in the CLN2 gene in the archival tissues and thereby convincingly demonstrated that these three children truly had classic late infantile neuronal ceroid lipofuscinosis (LINCL), now called CLN2. This archival study documents the possibilities to revalidate disease-specific original nosologic reports. Chloroquine is toxic to lysosomal enzymes and results in lysosomal storage. The material is autofluorescent and gives the ultrastructural pattern of curvilinear profiles, thus resembling classic late infantile NCL, representing a good experimental model. In humans chloroquine therapy may cause a myopathy (and retinopathy) and, as recently suggested, an encephalopathy marked by lysosomal accretion in several cell types including neurons. Immunohistochemically, SCMAS also accumulates, further strengthening morphologic similarity between LINCL and human chloroquine intoxication.

An Australasian diagnostic service for the neuronal ceroid lipofuscinoses

The neuronal ceroid lipofuscinoses (NCLs) are a family of related genetic disorders that together are believed to affect one child in every 12,500 births in the USA. Our laboratory has developed a diagnostic service for classical late infantile neuronal ceroid lipofuscinosis (LINCL) by assay of tripeptidyl-peptidase I (TPP-I) activity using the fluorogenic peptide substrate Ala-Ala-Phe aminomethylcoumarin, followed by a screen for three mutations in the CLN2 gene. In addition, we have also begun to offer a limited diagnostic service for the juvenile (JNCL) and infantile (INCL) forms of the disease on the basis of mutation analysis of the CLN3 and CLN1 genes, respectively. Retrospective analysis of Australasian patients with a clinical suspicion of NCL has revealed that six are affected by LINCL, six by JNCL and, to date, two by INCL. Mutation analysis of our LINCL patients has shown that the three screened mutations, namely, the nonsense mutation R208X and the splice mutations IVS5-1 G > C and IVS5-1 G > A, constitute 83% of alleles.

Heterogeneity of late-infantile neuronal ceroid lipofuscinosis

PURPOSE

Late-infantile neuronal ceroid lipofuscinosis (LINCL), an autosomal recessively inherited lysosomal storage disorder characterized by autofluorescent inclusions and rapid progression of neurodegeneration, is due to CLN2 gene mutations. However, CLN2 mutation analysis has failed to identify some clinically diagnosed "late-infantile" NCL cases. This study was conducted to further characterize genetic heterogeneity in families affected by LINCL.

METHODS

DNA mutations in the CLN1, CLN2, and CLN3 genes that underlie INCL (infantile NCL), LINCL, and JNCL (juvenile NCL), respectively, were studied with molecular analyses.

RESULTS

A total of 252 families affected by childhood NCL were studied. Of 109 families clinically diagnosed as having LINCL, 3 were determined to have either INCL or JNCL by identification of mutation(s) in CLN1 or CLN3. Six families diagnosed initially as having JNCL were found to have LINCL based on the finding of mutations in the CLN2 gene. In addition, several novel mutations were identified.

CONCLUSIONS

Clinical and genetic heterogeneity of LINCL was demonstrated in nine LINCL families studied.

Neuronal ceroid lipofuscinoses and possible pathogenic mechanism

The neuronal ceroid lipofuscinoses (NCLs) consist of eight autosomal recessively inherited storage disorders characterized by lysosomal inclusions of autofluorescent lipofuscins and rapid neurodegenerative progression. The NCLs include eight forms that result from genetic deficiency on genes CLN(1) to CLN(8), respectively: four classic forms with clinical onset at varying ages-infantile (INCL), late-infantile (LINCL), juvenile (JNCL), and adult (ANCL)-and four variants of late-infantile onset-the Finnish variant LINCL (fLINCL), Portuguese variant LINCL (pLINCL), Turkish variant LINCL (tLINCL), and progressive epilepsy with mental retardation (EPMR). The genes CLN(1) and CLN(2) have been characterized to encode lysosomal hydrolytic enzymes, but CLN(3), CLN(5), and CLN(8) encode transmembranous proteins with unknown function. Although clinical and pathological abnormalities have been recognized to be similar in all eight forms, the molecular mechanism explaining NCL pathogenesis remains unclear. In this review, the molecular basis for NCLs and a possible pathogenic mechanism are discussed.

Tripeptidyl-peptidase I deficiency in classical late-infantile neuronal ceroid lipofuscinosis brain tissue. Evidence for defective peptidase rather than proteinase activity

Brain tissue from patients with classical late-infantile neuronal ceroid lipofuscinosis (LINCL, an infantile form of Batten disease) is deficient in the lysosomal enzyme tripeptidyl-peptidase I (EC 3.4.14.9). The activities of other lysosomal enzymes are either increased or decreased. Tripeptidyl-peptidase I is a pepstatin-insensitive exo-tripeptidase, with little or no endo-proteolytic activity, that is active on small peptides but not on large proteins. Using haemoglobin and casein as substrates for proteolytic activity, we were unable to demonstrate any significant defect in pepstatin-sensitive or pepstatin-insensitive proteinase activity in brain tissue or cultured skin fibroblasts of LINCL patients. These observations suggest that the lysosomal storage of undegraded, small peptides in LINCL results from the absence of peptidase rather than proteinase activity.

Biochemical characterization of a lysosomal protease deficient in classical late infantile neuronal ceroid lipofuscinosis (LINCL) and development of an enzyme-based assay for diagnosis and exclusion of LINCL in human specimens and animal models

Classical late-infantile neuronal ceroid lipofuscinosis (LINCL), a progressive and fatal neurodegenerative disease of childhood, results from mutations in a gene (CLN2) that encodes a protein with significant sequence similarity to prokaryotic pepstatin-insensitive acid proteases. We have developed a sensitive protease activity assay that allows biochemical characterization of the CLN2 gene product in various human biological samples, including solid tissues (brain and chorionic villi), blood (buffy coat leukocytes, platelets, granulocytes, and mononuclear cells), and cultured cells (lymphoblasts, fibroblasts, and amniocytes). The enzyme has a pH optimum of 3.5 and is rapidly inactivated at neutral pH. A survey of fibroblasts and lymphoblasts demonstrated that lack of activity was associated with LINCL arising from mutations in the CLN2 gene but not other neuronal ceroid lipofuscinoses (NCLs), including the CLN6 variant LINCL, classical infantile NCL, classical juvenile NCL, and adult NCL (Kufs' disease). A study conducted using blood samples collected from classical LINCL families whose affliction was confirmed by genetic analysis indicates that the assay can distinguish homozygotes, heterozygotes, and normal controls and thus is useful for diagnosis and carrier testing. Analysis of archival specimens indicates that several specimens previously classified as LINCL have enzyme activity and thus disease is unlikely to arise from mutations in CLN2. Conversely, a specimen previously classified as juvenile NCL lacks pepinase activity and is associated with mutations in CLN2. In addition, several animals with NCL-like neurodegenerative symptoms [mutant strains of mice (nclf and mnd), English setter, border collie, and Tibetan terrier dogs, sheep, and cattle] were found to contain enzyme activity and are thus unlikely to represent models for classical LINCL. Subcellular fractionation experiments indicate that the CLN2 protein is located in lysosomes, which is consistent with its acidic pH optimum for activity and the presence of mannose 6-phosphate. Taken together, these findings indicate that LINCL represents a lysosomal storage disorder that is characterized by the absence of a specific protease activity.

Mutational analysis of the defective protease in classic late-infantile neuronal ceroid lipofuscinosis, a neurodegenerative lysosomal storage disorder

The late-infantile form of neuronal ceroid lipofuscinosis (LINCL) is a progressive and ultimately fatal neurodegenerative disease of childhood. The defective gene in this hereditary disorder, CLN2, encodes a recently identified lysosomal pepstatin-insensitive acid protease. To better understand the molecular pathology of LINCL, we conducted a genetic survey of CLN2 in 74 LINCL families. In 14 patients, CLN2 protease activities were normal and no mutations were identified, suggesting other forms of NCL. Both pathogenic alleles were identified in 57 of the other 60 LINCL families studied. In total, 24 mutations were associated with LINCL, comprising six splice-junction mutations, 11 missense mutations, 3 nonsense mutations, 3 small deletions, and 1 single-nucleotide insertion. Two mutations were particularly common: an intronic G-->C transversion in the invariant AG of a 3' splice junction, found in 38 of 115 alleles, and a C-->T transition in 32 of 115 alleles, which prematurely terminates translation at amino acid 208 of 563. An Arg-->His substitution was identified, which was associated with a late age at onset and protracted clinical phenotype, in a number of other patients originally diagnosed with juvenile NCL.

A lysosomal proteinase, the late infantile neuronal ceroid lipofuscinosis gene (CLN2) product, is essential for degradation of a hydrophobic protein, the subunit c of ATP synthase

The specific accumulation of the hydrophobic protein, subunit c of ATP synthase, in lysosomes from the cells of patients with the late infantile form of neuronal ceroid lipofuscinosis (LINCL) is caused by lysosomal proteolytic dysfunction. The defective gene in LINCL (CLN2 gene) has been identified recently. To elucidate the mechanism of lysosomal storage of subunit c, antibodies against the human CLN2 gene product (Cln2p) were prepared. Immunoblot analysis indicated that Cln2p is a 46-kDa protein in normal control skin fibroblasts and carrier heterozygote cells, whereas it was absent in cells from four patients with LINCL. RT-PCR analysis indicated the presence of mRNA for CLN2 in cells from the four different patients tested, suggesting a low efficiency of translation of mRNA or the production of the unstable translation products in these patient cells. Pulse-chase analysis showed that Cln2p was synthesized as a 67-kDa precursor and processed to a 46-kDa mature protein (t(1/2) = 1 h). Subcellular fractionation analysis indicated that Cln2p is localized with cathepsin B in the high-density lysosomal fractions. Confocal immunomicroscopic analysis also revealed that Cln2p is colocalized with a lysosomal soluble marker, cathepsin D. The immunodepletion of Cln2p from normal fibroblast extracts caused a loss in the degradative capacity of subunit c, but not the beta subunit of ATP synthase, suggesting that the absence of Cln2p provokes the lysosomal accumulation of subunit c.

A novel assay for lysosomal pepstatin-insensitive proteinase and its application for the diagnosis of late-infantile neuronal ceroid lipofuscinosis

A highly sensitive assay for mammalian lysosomal pepstatin-insensitive proteinase (LPIP) is described using a synthetic peptide substrate coupled to aminotrifluoromethyl coumarin (AFC). LPIP is an endocarboxyl proteinase which has specific sequence requirements of Phe-Phe around the carboxyl terminal. This HPLC based assay can detect patients suffering from late-infantile neuronal ceroid lipofuscinosis (LINCL) and also heterozygote carriers in cultured lymphoid cells and skin fibroblasts. None of the patients analyzed had detectable enzyme activity confirming the defective gene product, while carriers had about 50% activity when compared with the normal controls. Neurological controls comprised of patients with other neurodegenerative disorders have LPIP activities similar to normal controls. LPIP activity is also detectable in amniocytes and chorionic villi. Thus the assay reported can also be used for prenatal diagnosis of LINCL.

Classical late infantile neuronal ceroid lipofuscinosis fibroblasts are deficient in lysosomal tripeptidyl peptidase I

Tripeptidyl peptidase I (TPP-I) is a lysosomal enzyme that cleaves tripeptides from the N-terminus of polypeptides. A comparison of TPP-I amino acid sequences with sequences derived from an EST database suggested that TPP-I is identical to a pepstatin-insensitive carboxyl proteinase of unknown specificity which is mutated in classical late infantile neuronal ceroid lipofuscinosis (LINCL), a lysosomal storage disease. Both TPP-I and the carboxyl proteinase have an M(r) of about 46 kDa and are, or are predicted to be, resistant to inhibitors of the four major classes of proteinases. Fibroblasts from LINCL patients have less than 5% of the normal TPP-I activity. The activities of other lysosomal enzymes, including proteinases, are in the normal range. LINCL fibroblasts are also defective at degrading short polypeptides and this defect can be induced in normal fibroblasts by treatment with a specific inhibitor or TPP-I. These results suggest that the cell damage, especially neuronal, observed in LINCL results from the defective degradation and consequent lysosomal storage of small peptides.

Molecular basis of the neuronal ceroid lipofuscinoses: mutations in CLN1, CLN2, CLN3, and CLN5

The neuronal ceroid lipofuscinoses (NCLs), also referred to as Batten disease, are a group of neurodegenerative disorders characterised by the accumulation of an autofluorescent lipopigment in many cell types. Different NCL types are distinguished according to age of onset, clinical phenotype, ultrastructural characterisation of the storage material, and chromosomal location of the disease gene. At least eight genes underlie the NCLs, of which four have been isolated and mutations characterised: CLN1, CLN2, CLN3, CLN5. Two of these genes encode lysosomal enzymes, and two encode transmembrane proteins, at least one of which is likely to be in the lysosomal membrane. The basic defect in the NCLs appears to be associated with lysosomal function.