Deletion of the Caenorhabditis elegans homologues of the CLN3 gene, involved in human juvenile neuronal ceroid lipofuscinosis, causes a mild progeric phenotype

CLN3, at the crossroads of endocytic trafficking

The CLN3 gene was identified over two decades ago, but the primary function of the CLN3 protein remains unknown. Recessive inheritance of loss of function mutations in CLN3 are responsible for juvenile neuronal ceroid lipofuscinosis (Batten disease, or CLN3 disease), a fatal childhood onset neurodegenerative disease causing vision loss, seizures, progressive dementia, motor function loss and premature death. CLN3 is a multipass transmembrane protein that primarily localizes to endosomes and lysosomes. Defects in endocytosis, autophagy, and lysosomal function are common findings in CLN3-deficiency model systems. However, the molecular mechanisms underlying these defects have not yet been fully elucidated. In this mini-review, we will summarize the current understanding of the CLN3 protein interaction network and discuss how this knowledge is starting to delineate the molecular pathogenesis of CLN3 disease. Accumulating evidence strongly points towards CLN3 playing a role in regulation of the cytoskeleton and cytoskeletal associated proteins to tether cellular membranes, regulation of membrane complexes such as channels/transporters, and modulating the function of small GTPases to effectively mediate vesicular movement and membrane dynamics.

The contribution of multicellular model organisms to neuronal ceroid lipofuscinosis research

The NCLs (neuronal ceroid lipofuscinosis) are forms of neurodegenerative disease that affect people of all ages and ethnicities but are most prevalent in children. Commonly known as Batten disease, this debilitating neurological disorder is comprised of 13 different subtypes that are categorized based on the particular gene that is mutated (CLN1-8, CLN10-14). The pathological mechanisms underlying the NCLs are not well understood due to our poor understanding of the functions of NCL proteins. Only one specific treatment (enzyme replacement therapy) is approved, which is for the treating the brain in CLN2 disease. Hence there remains a desperate need for further research into disease-modifying treatments. In this review, we present and evaluate the genes, proteins and studies performed in the social amoeba, nematode, fruit fly, zebrafish, mouse and large animals pertinent to NCL. In particular, we highlight the use of multicellular model organisms to study NCL protein function, pathology and pathomechanisms. Their use in testing novel therapeutic approaches is also presented. With this information, we highlight how future research in these systems may be able to provide new insight into NCL protein functions in human cells and aid in the development of new therapies.

Modelling of Neuronal Ceroid Lipofuscinosis Type 2 in Dictyostelium discoideum Suggests That Cytopathological Outcomes Result from Altered TOR Signalling

The neuronal ceroid lipofuscinoses comprise a group of neurodegenerative disorders with similar clinical manifestations whose precise mechanisms of disease are presently unknown. We created multiple cell lines each with different levels of reduction of expression of the gene coding for the type 2 variant of the disease, Tripeptidyl peptidase (Tpp1), in the cellular slime mould Dictyostelium discoideum. Knocking down Tpp1 in Dictyostelium resulted in the accumulation of autofluorescent material, a characteristic trait of Batten disease. Phenotypic characterisation of the mutants revealed phenotypic deficiencies in growth and development, whilst endocytic uptake of nutrients was enhanced. Furthermore, the severity of the phenotypes correlated with the expression levels of Tpp1. We propose that the phenotypic defects are due to altered Target of Rapamycin (TOR) signalling. We show that treatment of wild type Dictyostelium cells with rapamycin (a specific TOR complex inhibitor) or antisense inhibition of expression of Rheb (Ras homologue enriched in the brain) (an upstream TOR complex activator) phenocopied the Tpp1 mutants. We also show that overexpression of Rheb rescued the defects caused by antisense inhibition of Tpp1. These results suggest that the TOR signalling pathway is responsible for the cytopathological outcomes in the Dictyostelium Tpp1 model of Batten disease.

Flunarizine rescues reduced lifespan in CLN3 triple knock-out Caenorhabditis elegans model of batten disease

CLN3 disease (Spielmeyer-Vogt-Sjogren-Batten disease, previously known as classic juvenile neuronal ceroid lipofuscinosis, NCL) is a pediatric-onset progressive neurodegenerative disease characterized by progressive vision loss, seizures, loss of cognitive and motor function, and early death. While no precise biochemical mechanism or therapies are known, the pathogenesis of CLN3 disease involves intracellular calcium accumulation that may trigger apoptosis. Our prior work in in vitro cell models of CLN3 deficiency suggested that FDA-approved calcium channel antagonists may have therapeutic value. To further evaluate the potential efficacy of this approach in an otherwise untreatable disorder, we sought to compare the therapeutic effects and underlying mechanisms in an animal model of CLN3 disease. Here, we used the well-characterized XT7 complete cln-3 knockout strain of C. elegans to evaluate the therapeutic efficacy of calcium channel antagonist therapy in a living animal model of Batten disease. Therapeutic effects of five calcium channel antagonists were evaluated on XT7 animal lifespan and in vivo mitochondrial physiology. Remarkably, maximal therapeutic efficacy in this model animal was observed with 1 μM flunarizine, the identical concentration previously identified in cell-based neuronal models of CLN3 disease. Specifically, flunarizine rescued the short lifespan of XT7 worms and prevented their pathophysiologic mitochondrial accumulation. These results confirm the treatment efficacy and dosing of flunarizine in cln-3 disease in a translational model organism. Clinical treatment trials in CLN3 human patients are now needed to test the dosing regimen and efficacy of flunarizine in individuals suffering with this otherwise untreatable and ultimately lethal neurologic disease.

Vision loss in juvenile neuronal ceroid lipofuscinosis (CLN3 disease)

Juvenile neuronal ceroid lipofuscinosis (JNCL; also known as CLN3 disease) is a devastating neurodegenerative lysosomal storage disorder and the most common form of Batten disease. Progressive visual and neurological symptoms lead to mortality in patients by the third decade. Although ceroid-lipofuscinosis, neuronal 3 (CLN3) has been identified as the sole disease gene, the biochemical and cellular bases of JNCL and the functions of CLN3 are yet to be fully understood. As severe ocular pathologies manifest early in disease progression, the retina is an ideal tissue to study in the efforts to unravel disease etiology and design therapeutics. There are significant discrepancies in the ocular phenotypes between human JNCL and existing murine models, impeding investigations on the sequence of events occurring during the progression of vision impairment. This review focuses on current understanding of vision loss in JNCL and discusses future research directions toward molecular dissection of the pathogenesis of the disease and associated vision problems in order to ultimately improve the quality of patient life and cure the disease.

[NCL in animal models]

Neuronal ceroid lipofuscinoses (NCL) are severe neurodegenerative diseases leading to early death. They belong to the group of lysosomal storage diseases. Epileptic seizures, dementia and motor deficits are frequent symptoms which are to be found prior to a total dismantling of personality and death. At present 10 subtypes of NCL can be distinguished from which the genetic defect is known in eight. The encoded proteins are soluble or membrane proteins whose function is still unclear in most cases. The investigation of the pathology and pathophysiology of NCL is highly dependent on animal models. Mouse models existing for all forms with a known genetic defect play a prominent role. Unfortunately, the retinal phenotype of some mouse models is milder than in humans rendering the appreciation of a positive therapeutic effect more difficult. Because of the severity of NCL, therapy strategies only established in a mouse model will be transferred to humans very quickly.

Caenorhabditis elegans as a model for lysosomal storage disorders

The nematode Caenorhabditis elegans is the simplest animal model available to study human disease. In this review, the worm homologues for the 58 human genes involved in lysosomal storage disorders and for 105 human genes associated with lysosomal function have been compiled. Most human genes had at least one worm homologue. In addition, the phenotypes of 147 mutants, in which these genes have been disrupted or knocked down, have been summarized and discussed. The phenotypic spectrum of worm models of lysosomal storage disorders varies from lethality to none obvious, with a large variety of intermediate phenotypes. The genetic power of C. elegans provides a means to identify genes involved in specific processes with relative ease. The overview of potential lysosomal phenotypes presented here might be used as a starting point for the phenotypic characterization of newly developed knock-out models or for the design of genetic screens selecting for loss or gain of suitable knock-out model phenotypes. Screens for genes involved in lysosomal biogenesis and function have been performed successfully resulting in the cup and glo mutants, but screens involving subtle phenotypes are likely to be difficult.

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

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

Characterizing pathogenic processes in Batten disease: Use of small eukaryotic model systems

The neuronal ceroid lipofuscinoses (NCLs) are neurodegenerative disorders. Nevertheless, small model organisms, including those lacking a nervous system, have proven invaluable in the study of mechanisms that underlie the disease and in studying the functions of the conserved proteins associated to each disease. From the single-celled yeast, Saccharomyces cerevisiae and Schizosaccharomyces pombe, to the worm, Caenorhabditis elegans and the fruitfly, Drosophila melanogaster, biochemical and, in particular, genetic studies on these organisms have provided insight into the NCLs.

From genes to systems: new global strategies for the characterization of NCL biology

Neuronal ceroid lipofuscinoses (NCL) are rare neurological disorders with a uniform phenotype, caused by mutations in seven known genes. NCL provide a unique model to characterize molecular pathways critical for normal neuronal development and pathological neuronal degeneration. Systems biology based approach utilizes the rapidly developing tools of genomics, proteomics, lipidomics and metabolomics and aims at thorough understanding of the functions of cells, tissues and whole organisms by molecular analysis and biocomputing-assisted modeling. The systems level understanding of NCL is now possible by utilizing different model organisms. Initial work has revealed disturbed metabolic pathways in several NCL disorders and most analyses have utilized the infantile (INCL/CLN1) and juvenile (JNCL/CLN3) disease modeling and utilized mainly human and mouse samples. To date, the data obtained from transcript and lipidomic profiling has pinpointed the role of lipid metabolism and synaptic function in the infantile NCL. Changes in glutamate utilization and amino acid metabolism have been a common theme emerging from the transcript and metabolite profiling of the juvenile NCL. Further experimental models are being developed and systematic sample collection as well as data integration projects are needed. The combined analyses of the global information should provide means to expose all the NCL-associated molecular pathways.