Prosegment of tripeptidyl peptidase I is a potent, slow-binding inhibitor of its cognate enzyme.

Tripeptidyl peptidase I (TPP I) is the first mammalian representative of a family of pepstatin-insensitive serine-carboxyl proteases, or sedolisins. The enzyme acts in lysosomes, where it sequentially removes tripeptides from the unmodified N terminus of small, unstructured polypeptides. Naturally occurring mutations in TPP I underlie a neurodegenerative disorder of childhood, classic late infantile neuronal ceroid lipofuscinosis (CLN2). Generation of mature TPP I is associated with removal of a long prosegment of 176 amino acid residues from the zymogen. Here we investigated the inhibitory properties of TPP I prosegment expressed and isolated from Escherichia coli toward its cognate protease. We show that the TPP I prosegment is a potent, slow-binding inhibitor of its parent enzyme, with an overall inhibition constant in the low nanomolar range. We also demonstrate the protective effect of the prosegment on alkaline pH-induced inactivation of the enzyme. Interestingly, the inhibitory properties of TPP I prosegment with the introduced classic late infantile neuronal ceroid lipofuscinosis disease-associated mutation, G77R, significantly differed from those revealed by wild-type prosegment in both the mechanism of interaction and the inhibitory rate. This is the first characterization of the inhibitory action of the sedolisin prosegment.

Increased levels of carbonic anhydrase II in the developing Down syndrome brain.

By using a proteomic approach, we found increased levels of carbonic anhydrase II (CA II) in the brain of Ts65Dn mice, a mouse model for Down syndrome (DS). Further immunoblot analyses showed that the levels of CA II are increased not only in the brain of adult Ts65Dn mice but also in the brain of infants and young children with DS. Cellular localization of the enzyme in human brain, predominantly in the oligodendroglia and primitive vessels in fetal brain and in the oligodendroglia and some GABAergic neurons postnatally, was similar in DS subjects and controls. Given the role of CA II in regulation of electrolyte and water balance and pH homeostasis, up-regulation of CA II may reflect a compensatory mechanism mobilized in response to structural/functional abnormalities in the developing DS brain. However, this up-regulation may also have an unfavorable effect by increasing susceptibility to seizures of children with DS.

ER and oxidative stresses are common mediators of apoptosis in both neurodegenerative and non-neurodegenerative lysosomal storage disorders and are alleviated by chemical chaperones.

It is estimated that more than 40 different lysosomal storage disorders (LSDs) cumulatively affect one in 5000 live births, and in the majority of the LSDs, neurodegeneration is a prominent feature. Neuronal ceroid lipofuscinoses (NCLs), as a group, represent one of the most common (one in 12,500 births) neurodegenerative LSDs. The infantile NCL (INCL) is the most devastating neurodegenerative LSD, which is caused by inactivating mutations in the palmitoyl-protein thioesterase-1 (PPT1) gene. We previously reported that neuronal death by apoptosis in INCL, and in the PPT1-knockout (PPT1-KO) mice that mimic INCL, is at least in part caused by endoplasmic reticulum (ER) and oxidative stresses. In the present study, we sought to determine whether ER and oxidative stresses are unique manifestations of INCL or they are common to both neurodegenerative and non-neurodegenerative LSDs. Unexpectedly, we found that ER and oxidative stresses are common manifestations in cells from both neurodegenerative and non-neurodegenerative LSDs. Moreover, all LSD cells studied show extraordinary sensitivity to brefeldin-A-induced apoptosis, which suggests pre-existing ER stress conditions. Further, we uncovered that chemical disruption of lysosomal homeostasis in normal cells causes ER stress, suggesting a cross-talk between the lysosomes and the ER. Most importantly, we found that chemical chaperones that alleviate ER and oxidative stresses are also cytoprotective in all forms of LSDs studied. We propose that ER and oxidative stresses are common mediators of apoptosis in both neurodegenerative and non-neurodegenerative LSDs and suggest that the beneficial effects of chemical/pharmacological chaperones are exerted, at least in part, by alleviating these stress conditions.

Carbonic anhydrase II in the developing and adult human brain.

Carbonic anhydrase II (CA II) is one of 14 isozymes of carbonic anhydrases, zinc metalloenzymes that catalyze the reversible hydration of carbon dioxide to bicarbonate. Mutations in CA II in humans lead to osteopetrosis with renal tubular acidosis and cerebral calcifications, a disorder often associated with mental retardation. Recently, new avenues in CA II research have opened as a result of discoveries that the enzyme increases bicarbonate and proton fluxes and may play an important role in brain tissue. In the human brain, CA II was localized to oligodendrocytes, myelin, and choroid plexus epithelium. Because this conclusion was based on a few fragmentary reports, we analyzed in more detail the expression of the enzyme in human telencephalon. By immunoblotting, we found a gradual increase in CA II levels from 17 weeks' gestation to childhood and adolescence. By immunohistochemistry, CA II was found to be present not only in oligodendrocytes and choroid plexus epithelium (declining with aging in both these locations), but also in a subset of neurons mostly with GABAergic phenotype, in a few astrocytes, and transiently during brain development in the endothelial cells of microvessels. The enzyme also occurred in oligodendrocyte processes in contact with myelinating axons, myelin sheaths, and axolemma, but was either absent or appeared in minute amounts in compact myelin. These findings suggest the possible involvement of CA II in a wide spectrum of biologic processes in the developing and adult human brain and may contribute to better understanding of the pathogenesis of cerebral calcifications and mental retardation caused by CA II deficiency.

Genotype-phenotype analyses of classic neuronal ceroid lipofuscinosis (NCLs): genetic predictions from clinical and pathological findings.

OBJECTIVE: Genotype-phenotype associations were studied in 517 subjects clinically affected by classical neuronal ceroid lipofuscinosis (NCL). METHODS: Genetic loci CLN1-3 were analyzed in regard to age of onset, initial neurological symptoms, and electron microscope (EM) profiles. RESULTS: The most common initial symptom leading to a clinical evaluation was developmental delay (30%) in NCL1, seizures (42.4%) in NCL2, and vision problems (53.5%) in NCL3. Eighty-two percent of NCL1 cases had granular osmiophilic deposits (GRODs) or mixed-GROD-containing EM profiles; 94% of NCL2 cases had curvilinear (CV) or mixed-CV-containing profiles; and 91% of NCL3 had fingerprint (FP) or mixed-FP-containing profiles. The mixed-type EM profile was found in approximately one-third of the NCL cases. DNA mutations within a specific CLN gene were further correlated with NCL phenotypes. Seizures were noticed to associate with common mutations 523G>A and 636C>T of CLN2 in NCL2 but not with common mutations 223G>A and 451C>T of CLN1 in NCL1. Vision loss was the initial symptom in all types of mutations in NCL3. Surprisingly, our data showed that the age of onset was atypical in 51.3% of NCL1 (infantile form) cases, 19.7% of NCL2 (late-infantile form) cases, and 42.8% of NCL3 (juvenile form) cases. CONCLUSION: Our data provide an overall picture regarding the clinical recognition of classical childhood NCLs. This may assist in the prediction and genetic identification of NCL1-3 via their characteristic clinical features.

Confronting the issues of therapeutic misconception, enrollment decisions, and personal motives in genetic medicine-based clinical research studies for fatal disorders.

Genetic medicine-based therapies have unlocked the potential for ameliorating diseases previously considered inevitably fatal. Inherent in the clinical trials of genetic medicines are ethical issues of therapeutic misconception, enrollment decisions as they relate to the risks and benefits of research, and the complex relationships among funding sources, investigators, and the families of affected individuals. The purpose of this paper is to help define these complex issues relevant to the use of genetic medicines and to describe the strategy we have used to confront these issues in a phase I trial of adeno-associated virus-mediated gene transfer to the central nervous system of children with late infantile neuronal ceroid lipofuscinosis (LINCL), a fatal lysosomal storage disease associated with progressive neurodegeneration and death by mid-childhood. Our approach to these challenges should provide a useful paradigm for investigators initiating other genetic medicine- based studies to treat inevitably fatal diseases.

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.

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.

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.

Current state of clinical and morphological features in human NCL.

The neuronal ceroid lipofuscinoses (NCL) are a large group of autosomal recessive lysosomal storage disorders with both enzymatic deficiency and structural protein dysfunction. Previously, diagnosis of NCL was based on age at onset and clinicopathological (C-P) findings described 4 forms, classified as infantile (INCL) (2), late-infantile (LINCL) (5), juvenile (JNCL) (6), and adult (ANCL) (12). Most patients with NCL have progressive ocular and cerebral dysfunction, including cognitive/motor dysfunction and uncontrolled seizures. After reviewing 520 patients with NCL, we found that about 104 (20%) did not fit this classification of NCL. With further research, 4 additional forms have been recognized: Finnish (13), Gypsy/Indian (14), Turkish (15)--variants of LINCL, and Northern epilepsy (16), also known as progressive epilepsy with mental retardation. These eight NCL forms resulted from 151 different mutations in genes CLN1 to CLN8 causing different phenotypes (http://www.ucl.ac.uk/ncl). The genes CLN1 and CLN2 encode lysosomal palmitoyl protein thioesterase and tripeptidyl peptidase 1. The diagnosis of NCL is based on clinicopathological (C-P) findings, enzymatic assay, and molecular genetic testing. Ultrastructural studies must be performed to confirm the presence and nature of lysosomal storage material (fingerprint or curvilinear profiles, or granular osmiophilic deposits) before doing biochemical testing. Pheno/genotypic correlation studies are discussed.

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.

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

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

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.