Sphingolipid activator proteins in the neuronal ceroid-lipofuscinoses: an immunological study

Aberrant upregulation of the glycolytic enzyme PFKFB3 in CLN7 neuronal ceroid lipofuscinosis

CLN7 neuronal ceroid lipofuscinosis is an inherited lysosomal storage neurodegenerative disease highly prevalent in children. CLN7/MFSD8 gene encodes a lysosomal membrane glycoprotein, but the biochemical processes affected by CLN7-loss of function are unexplored thus preventing development of potential treatments. Here, we found, in the Cln7∆ex2 mouse model of CLN7 disease, that failure in autophagy causes accumulation of structurally and bioenergetically impaired neuronal mitochondria. In vivo genetic approach reveals elevated mitochondrial reactive oxygen species (mROS) in Cln7∆ex2 neurons that mediates glycolytic enzyme PFKFB3 activation and contributes to CLN7 pathogenesis. Mechanistically, mROS sustains a signaling cascade leading to protein stabilization of PFKFB3, normally unstable in healthy neurons. Administration of the highly selective PFKFB3 inhibitor AZ67 in Cln7∆ex2 mouse brain in vivo and in CLN7 patients-derived cells rectifies key disease hallmarks. Thus, aberrant upregulation of the glycolytic enzyme PFKFB3 in neurons may contribute to CLN7 pathogenesis and targeting PFKFB3 could alleviate this and other lysosomal storage diseases.

Neuronal ceroid lipofuscinosis in the Russian population: Two novel mutations and the prevalence of heterozygous carriers

Background

Neuronal ceroid lipofuscinoses (NCLs) are a group of neurodegenerative disorders characterized by an accumulation of lipofuscin in the body's tissues. NCLs are associated with variable age of onset and progressive symptoms including seizures, psychomotor decline, and loss of vision.

Methods

We describe the clinical and molecular characteristics of four Russian patients with NCL (one female and three males, with ages ranging from 4 to 5 years). The clinical features of these patients include cognitive and motor deterioration, seizures, stereotypies, and magnetic resonance imaging signs of brain atrophy. Exome sequencing was performed to identify the genetic variants of patients with NCL. Additionally, we tested 6,396 healthy Russians for NCL alleles.

Results

We identified five distinct mutations in four NCL‐associated genes of which two mutations are novel. These include a novel homozygous frameshift mutation in the CLN6 gene, a compound heterozygous missense mutation in the KCTD7 gene, and previously known mutations in KCTD7, TPP1, and MFSD8 genes. Furthermore, we estimated the Russian population carrier frequency of pathogenic and likely pathogenic variants in 13 genes associated with different types of NCL.

Conclusion

Our study expands the spectrum of mutations in lipofuscinosis. This is the first study to describe the molecular basis of NCLs in Russia and has profound and numerous clinical implications for diagnosis, genetic counseling, genotype–phenotype correlations, and prognosis.

The juvenile Batten disease protein, CLN3, and its role in regulating anterograde and retrograde post-Golgi trafficking

Loss-of-function mutations in CLN3 are responsible for juvenile-onset neuronal ceroid lipofuscinosis (JNCL), or Batten disease, which is an incurable lysosomal disease that manifests with vision loss, followed by seizures and progressive neurodegeneration, robbing children of motor skills, speech and cognition, and eventually leading to death in the second or third decade of life. Emerging clinical evidence points to JNCL pathology outside of the CNS, including the cardiovascular system. The CLN3 gene encodes an unusual transmembrane protein, CLN3 or battenin, whose elusive function has been the subject of intense study for more than 10 years. Owing to the detailed characterization of a large number of disease models, our knowledge of CLN3 protein function is finally coming into focus. This review will describe the most current understanding of CLN3 structure, function and dysfunction in JNCL.

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. Recent advances

The neuronal ceroid lipofuscinoses (NCLs) represent a group of neurodegenerative disorders characterised by progressive visual failure, neurodegeneration, epilepsy and the accumulation of an autofluorescent lipopigment in neurons and other cells. The main childhood subtypes are infantile (INCL;CLN1), classical late infantile (LINCL;CLN2) and juvenile NCL (JNCL;CLN3), distinguished on the basis of age of onset, clinical course and ultrastructural morphology, and recently genetic analysis. In addition several variant forms of the disease complex have been described as well as a rare adult onset form. Advances in both genetics and biochemistry have led to the identification of the genes for the three main subtypes of childhood NCL and their corresponding protein products and to mapping of two additional genes for two variant forms. The disease causing genes in both INCL and classical LINCL have been shown to encode lysosomal enzymes whilst the JNCL gene codes for a protein whose function is as yet unknown.

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.

The neuronal ceroid-lipofuscinoses

The neuronal ceroid-lipofuscinoses (NCLs) collectively constitute the most common group of neurodegenerative diseases in childhood and usually show an autosomal recessive mode of inheritance. Despite varying ages of onset and clinical course characterized in most instances by progressive mental and motor deterioration, blindness, epileptic seizures, and premature death, all forms of NCL show unifying histopathological features. There is accumulation of autofluorescent, periodic acid-Schiff-, and Sudan black B-positive granules that are resistant to lipid solvents in the cytoplasm of most nerve cells and. to a lesser degree, of many other cell types. The storage process is associated with progressive and selective neuronal loss and gliosis with secondary white matter lesions. The ultrastructure of the storage deposits varies between different forms of NCL and, along with the age of onset, has provided the basis for the traditional classification of NCLs. Recent molecular genetic findings have established that defects in at least 7 different genes underlie the various forms of NCL. The purpose of this paper is to provide an overview of the NCLs, review recent molecular genetic and biochemical findings, and discuss their impact on our views on the classification and pathogenesis of these devastating brain disorders.

Cellular pathology and pathogenic aspects of neuronal ceroid lipofuscinoses

Lysosomal accumulation of autofluorescent, ceroid lipopigment material in various tissues and organs is a common feature of the neuronal ceroid lipofuscinoses (NCLs). However, recent clinicopathologic and genetic studies have evidenced that NCLs encompass a group of highly heterogeneous disorders. In five of the eight NCL variants distinguished at present, genes associated with the disease process have been isolated and characterized (CLN1, CLN2, CLN3, CLN5, CLN8). Only products of two of these genes, CLN 1 and CLN2, have structural and functional properties of lysosomal enzymes. Nevertheless, according to the nature of the material accumulated in the lysosomes, NCLs in humans as well as natural animal models of these disorders can be divided into two major groups: those characterized by the prominent storage of saposins A and D, and those showing the predominance of subunit c of mitochondrial ATP synthase accumulation. Thus, taking into account the chemical character of the major component of the storage material, NCLs can be classified currently as proteinoses. Of importance, although lysosomal storage material accumulates in NCL subjects in various organs, only brain tissue shows severe dysfunction and cell death, another common feature of the NCL disease process. However, the relation between the genetic defects associated with the NCL forms, the accumulation of storage material, and tissue damage is still unknown. This chapter introduces the reader to the complex pathogenesis of NCLs and summarizes our current knowledge of the potential consequences of the genetic defects of NCL-associated proteins on the biology of the cell.

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

The neuronal ceroid lipofuscinoses (NCLs) constitute a group of neurodegenerative storage diseases characterized by progressive psychomotor retardation, blindness and premature death. Pathologically, there is accumulation of autofluorescent material in lysosome-derived organelles in a variety of cell types, but neurons in the central nervous system appear to be selectively affected and undergo progressive death. In this report we show that a novel form of NCL, congenital ovine NCL, is caused by a deficiency in the lysosomal aspartyl proteinase cathepsin D. A single nucleotide mutation in the cathepsin D gene results in conversion of an active site aspartate to asparagine, leading to production of an enzymatically inactive but stable protein. This results in severe cerebrocortical atrophy and early death, providing strong evidence for an important role of cathepsin D in neuronal development and/or homeostasis.

Northern epilepsy: a novel form of neuronal ceroid-lipofuscinosis

Northern epilepsy is an autosomal recessive childhood onset epilepsy syndrome, clinically characterized by generalized tonic-clonic seizures with onset at 5 to 10 years of age and subsequent slowly progressive mental deterioration. The patients may reach 50 or 60 years of age. A mutation responsible for the disease has recently been identified in a novel gene on chromosome 8p23, encoding a putative membrane protein with an unknown function. The present study, based on three autopsied patients, is the first neuropathological analysis of the disease, and showed intraneuronal accumulation of cytoplasmic autofluorescent granules. The granules were strongly stained by the Luxol fast blue, periodic acid-Schiff, and Sudan black B methods in paraffin sections, and were immunoreactive for subunit c of the mitochondrial ATP synthase and sphingolipid activator proteins A and D. The intraneuronal storage was highly selective: the third layer of the isocortex and the hippocampal CA2, CA3, and CA4 sectors were severely affected, while other layers of the isocortex, the CA1 sector, and the cerebellar cortex were only minimally involved. The membrane-bound storage cytosomes showed a curvilinear ultrastructure with admixture of some granular components. Western blotting and N-terminal sequence analysis of purified storage material identified subunit c as the major component. These findings establish Northern epilepsy as a new form of neuronal ceroid-lipofuscinosis with an exceptionally protracted course.

Batten disease: four genes and still counting

The neuronal ceroid lipofuscinoses (NCLs, also known as Batten disease) are the most common childhood neurodegenerative disease. They are a group of inherited neurodegenerative disorders characterized by the accumulation of autofluorescent storage material in many cell types. Clinical features include seizures, psychomotor deterioration, and blindness, the ages and order of onset of which differ for each NCL type. An increasing number of subtypes caused by mutations in different genes are now recognized. With the advent of molecular genetics the basic genetic defect underlying each NCL phenotype is being determined, thus shedding light on the molecular basis of the NCLs and opening the way for the development of effective treatment. Four genes have been identified to date. The function of two of these is known and suggests that the primary defect in the NCLs lies in lysosomal proteolysis, the first example of this type of disease. However, since the function of the other two genes remains elusive, and at least four more genes remain to be identified, the molecular basis underlying the NCLs may be more complex than originally predicted.

The dystrophic retina in multisystem disorders: the electroretinogram in neuronal ceroid lipofuscinoses

The neuronal ceroid lipofuscinoses (NCL) are neurodegenerative disorders with psychomotor deterioration, seizures, visual failure and premature death, all associated with abnormal storage of lipoproteins within lysosomes. The most common forms of NCL are an infantile form (INCL, CLN1), a late infantile form (LINCL, CLN2) and a juvenile onset form (JNCL, CLN3). The electroretinogram (ERG) is abnormal early in all three of these forms and eventually is totally ablated. The purpose of this report is to describe the ERG in INCL, LINCL and JNCL. The ERGs of 7 patients who were examined by the author over the past 15 years were reviewed. Ganzfeld ERG responses were recorded using the ISCEV standard protocol and an intensity response series over a 3.7 log unit range. The earliest ERG manifestation of INCL is a marked loss of the scotopic and photopic b-wave with relative preservation of the a-wave; this defect, which was evident for both rods and cones, suggests preservation of photoreceptor outer segment function with severe disturbance of transmission of the signal to the second-order neuron, the bipolar cells. For LINCL, the rod responses were mildly abnormal but more preserved than in INCL or JNCL. The cone b-wave amplitudes in patients with early LINCL were severely subnormal with prolonged implicit times. Patients with JNCL invariably showed severe to profound ERG abnormalities when first tested, with essentially no rod-mediated activity and marked loss of a-wave amplitudes with even greater loss of b-wave amplitudes, creating electronegative configuration waveforms. Differences in the ERG responses were thus found that provide further clues to the earliest site of pathology within the retina.

Lysosomal storage diseases of animals: an essay in comparative pathology

A wide variety of inherited lysosomal hydrolase deficiencies have been reported in animals and are characterized by accumulation of sphingolipids, glycolipids, oligosaccharides, or mucopolysaccharides within lysosomes. Inhibitors of a lysosomal hydrolase, e.g., swainsonine, may also induce storage disease. Another group of lysosomal storage diseases, the ceroid-lipofuscinoses, involve the accumulation of hydrophobic proteins, but their pathogenesis is unclear. Some of these diseases are of veterinary importance, and those caused by a hydrolase deficiency can be controlled by detection of heterozygotes through the gene dosage phenomenon or by molecular genetic techniques. Other of these diseases are important to biomedical research either as models of the analogous human disease and/or through their ability to help elucidate specific aspects of cell biology. Some of these models have been used to explore possible therapeutic strategies and to define their limitations and expectations.

Accumulation of sphingolipid activator proteins (SAPs) A and D in granular osmiophilic deposits in miniature Schnauzer dogs with ceroid-lipofuscinosis

The neuronal ceroid-lipofuscinoses (NCL, Batten disease) are fatal inherited neurodegenerative diseases of children characterized by retinal and brain atrophy and the accumulation of electron-dense storage bodies in cells. Mutations in different genes underlie different major forms. The infantile disease (CLN-1, McKusick 256730) is distinguished by the storage of the sphingolipid activator proteins (SAPs) A and D in distinctive granular osmiophilic deposits (GRODs). This contrasts with the other major forms, where subunit c of mitochondrial ATP synthase is stored in various multilamellar profiles. Ceroid-lipofuscinoses also occur in dogs, including a form in miniature Schnauzers with distinctive granular osmiophilic deposit-like storage bodies. Antisera to SAPs A and D reacted to these storage bodies in situ. The presence of SAP D was confirmed by Western blotting and of SAP A by protein sequencing. Neither subunit c of mitochondrial ATP synthase nor of vacuolar ATPase is stored. This suggests that there are two families of ceroid-lipofuscinoses, the subunit c-storing forms, and those in which SAPs A and D, and perhaps other proteins, accumulate. Further work is required to determine whether other forms with granular osmiophilic deposits belong to the latter class and the genetic relationships between them and the human infantile disease.

The mannosidoses and ceroid-lipofuscinoses: experimental studies on two types of storage disease

alpha-Mannosidosis of Angus calves was studied both for its veterinary importance and as a model of analogous human lysosomal storage diseases. This study facilitated a similar study in Australia on Swainsona spp. intoxication of livestock in which the toxic principle was shown to be an indolizidine alkaloid, Swainsonine. These genetic and acquired alpha-mannosidoses are compared with beta-mannosidosis. Collectively the study has helped the understanding of the processes of glycosylation and catabolism of glycoproteins. An experiment of nature involving an alpha-mannosidosis chimeric calf born co-twin to a normal calf helped to define the expectations and limitations of bone marrow transplants in this type of storage disease in humans. The inherited ceroid-lipofuscinoses (Batten disease) were studied in an ovine model. Isolation and analyses of the fluorescent accumulated lipopigment denied the dogma of lipid peroxidation current in the 1970s and 1980s. It was shown that in this, and analogous diseases in humans, the dominantly accumulated species was the very hydrophobic protein, subunit c of mitochondrial ATP synthase. Contrary to the adage that this should reflect a disorder of lysosomal proteolysis, there is accumulating evidence that the primary defect resides in mitochondria. Because of its hydrophobic nature, subunit c forms paracrystaline complexes which appear resistant to proteolysis within the lysosomal apparatus.

The neuronal ceroid-lipofuscinoses

The neuronal ceroid lipofuscinoses (NCL) are a relatively frequent group of progressive neurodegenerative disorders in children with similar, but not identical, clinical and morphological features, entailing different clinical groups, some of which have been found to represent different genetic entities, ie, infantile (INCL) or CLN1, late-infantile (LINCL) or CLN2, juvenile (JNCL) or CLN3, and a Finnish variant of LINCL or CLN5. Within the clinical pentad are included seizures, motor disturbances, visual impairment, dementia, and familial occurrence in an autosomal-recessive fashion. The ultrastructure of accruing lipopigments is diagnostically required to recognize an individual patient's NCL by showing granular lipopigments in INCL, curvilinear profiles (with or without fingerprint profiles) in LINCL and fingerprint profiles (with or without curvilinear profiles) in JNCL. Identification of genes for INCL and JNCL, together with electron microscopy in LINCL, allows safe prenatal diagnosis which is still impossible by biochemical techniques, unlike other lysosomal disorders. However, both cause and pathogenesis of the individual forms of NCL are still unknown, and therapy is gravely insufficient.

Brain lysosomal hydrolases in neuronal ceroid-lipofuscinoses

Although the neuronal ceroid-lipofuscinoses (NCLs) are often referred to as lysosomal storage disorders, information on brain lysosomal hydrolases in NCLs is not available. We have determined the specific activities of several acid hydrolases in postmortem brain gray matter of infantile (INCL), late infantile (LINCL), juvenile (JNCL), and adult (ANCL) forms of NCL, patients affected with other neurological disorders (ON), and normal controls. The specific activities of beta-hexosaminidase A and B were significantly high in JNCL gray matter, whereas in LINCL, the increase is significant only in beta-hexosaminidase compared to the controls. A significant increase in the activities of alpha-mannosidase, beta-glucuronidase, and acid phosphatase was also observed in LINCL and JNCL patients compared to the control values. beta-galactosidase activity was also found to be elevated in JNCL brains over the controls. In contrast, activities of beta-glucosidase and sialidase appeared to be lowered in INCL and LINCL. On the other hand, alpha-fucosidase, beta-mannosidase, and sulfatase were unaffected in NCLs brains. Thus, the present data indicate NCLs related abnormalities in some of the acid hydrolases in brain gray matter, which are primarily glycoproteins of lysosomal origin. These data in conjuction with the reported association of sphingolipid activator proteins (SAP) A and D and lysosomal glycoproteins with NCL storage bodies imply abberations in the glycoconjugate metabolism and lysosomal function.

Neuronal ceroid-lipofuscinosis--late-infantile or Jansky-Bielschowsky type--revisited

The tissues from three patients with late-infantile NCL originally described by Max Bielschowsky became available to apply modern techniques such as fluorescence microscopy, electron microscopy and immunohistochemistry. While regular tinctorial preparations of the tissues documented a neuronal storage disorder in all three patients' tissues, the accumulated material proved to be autofluorescent, showed the ultrastructure of curvilinear lipopigments, and reacted strongly with an antibody against the subunit-C of mitochondrial ATP synthase, a major component of lipopigments in NCL and also with an antibody against sphingolipid activator proteins. Thus, these modern morphological techniques demonstrated that the originally described three siblings with late-infantile "amaurotic familial idiocy" really had neuronal ceroid-lipofuscinosis of the late-infantile or Jansky-Bielschowsky type, according to current diagnostic criteria. This type of archival study may also contribute to the mosaic of medical history.

Recent advances in the molecular genetics of the neuronal ceroid lipofuscinoses

Major advances in the molecular genetic analysis of the neuronal ceroid lipofuscinoses (NCL) have recently been made: the genes for two major types have been identified and the chromosomal location for a third defined. CLN1, the gene for infantile NCL (Santavuori-Haltia disease) encodes palmitoyl protein thioesterase (PPT). Most patients (75% of disease chromosomes) have the same point mutation. In contrast, CLN3, the gene for juvenile NCL (Batten or Spielmeyer-Vogt-Sjögren disease) is not a previously known gene, nor does its product display homology to any previously described proteins. The same 1 kb genomic deletion is present in the majority of patients (81% of disease chromosomes). CLN5, the gene for Finnish variant late infantile NCL, has been mapped to 13q and should be identified in the near future. The gene for late-infantile NCL (Jansky-Bielschowsky disease) has not yet been localized to a chromosome despite intensive research. It is likely that this type of NCL is caused by mutations in more than one gene each resulting in the same phenotype.