Intermediate levels of neuronal palmitoyl-protein Delta-9 desaturase in heterozygotes for murine Batten disease

The CLN3 gene and protein: What we know

Background

One of the most important steps taken by Beyond Batten Disease Foundation in our quest to cure juvenile Batten (CLN3) disease is to understand the State of the Science. We believe that a strong understanding of where we are in our experimental understanding of the CLN3 gene, its regulation, gene product, protein structure, tissue distribution, biomarker use, and pathological responses to its deficiency, lays the groundwork for determining therapeutic action plans.

Objectives

To present an unbiased comprehensive reference tool of the experimental understanding of the CLN3 gene and gene product of the same name.

Methods

BBDF compiled all of the available CLN3 gene and protein data from biological databases, repositories of federally and privately funded projects, patent and trademark offices, science and technology journals, industrial drug and pipeline reports as well as clinical trial reports and with painstaking precision, validated the information together with experts in Batten disease, lysosomal storage disease, lysosome/endosome biology.

Results

The finished product is an indexed review of the CLN3 gene and protein which is not limited in page size or number of references, references all available primary experiments, and does not draw conclusions for the reader.

Conclusions

Revisiting the experimental history of a target gene and its product ensures that inaccuracies and contradictions come to light, long‐held beliefs and assumptions continue to be challenged, and information that was previously deemed inconsequential gets a second look. Compiling the information into one manuscript with all appropriate primary references provides quick clues to which studies have been completed under which conditions and what information has been reported. This compendium does not seek to replace original articles or subtopic reviews but provides an historical roadmap to completed works.

The neuronal ceroid-lipofuscinoses

The neuronal ceroid-lipofuscinoses (NCL's, Batten disease) represent a group of severe neurodegenerative diseases, which mostly present in childhood. The phenotypes are similar and include visual loss, seizures, loss of motor and cognitive function, and early death. At autopsy, there is massive neuronal loss with characteristic storage in remaining neurons. Neurons appear to die because of increased rates of apoptosis and altered autophagy. Ten genes have been identified so far that result in an NCL (CLN1-10). The most common forms are CLN1, CLN2, and CLN3, which were previously known as Infantile, Late-Infantile, and Juvenile NCL's, respectively. CLN1 and CLN2 result from mutations in soluble lysosomal enzymes palmitoyl-protein thioesterase (PPT) and tripeptidyl peptidase 1 (TPP1), which can be measured in white blood cells for clinical diagnosis. Molecular diagnostic testing is routinely available for CLN1, CLN2, and CLN3. Sequencing of other NCL genes may be required to establish a diagnosis when the common forms are ruled out. The pathogenesis of NCL neuronal loss resulting from loss of function of any of the NCL gene products remains unknown and no treatment options are presently available.

Mouse models of neuronal ceroid lipofuscinoses: useful pre-clinical tools to delineate disease pathophysiology and validate therapeutics

The neuronal ceroid lipofuscinoses (NCL, also known as Batten disease) is a devastating neurodegenerative diseases caused by mutations in either soluble enzymes or membrane-associated structural proteins that result in lysosome dysfunction. Different forms of NCL were defined initially by age of onset, affected population and/or type of storage material but collectively represent the most prevalent pediatric hereditary neurovisceral storage disorder. Specific gene mutations are now known for each subclass of NCL in humans that now largely define the disease: cathepsin D (CTSD) for congenital (CLN10 form); palmitoyl protein thioesterase 1 (PPT1) for infantile (CLN1 form); tripeptidyl peptidase 1 (TPP1) for classic late infantile (CLN2 form); variant late infantile-CLN5, CLN6 or CLN8 for variant late infantile forms; and CLN3 for juvenile (CLN3 form). Several mouse models of NCL have been developed, or in some cases exist sporadically, that exhibit mutations producing a progressive neurodegenerative phenotype similar to that observed in human NCL. The study of these mouse models of NCL has dramatically advanced our knowledge of NCL pathophysiology and in some cases has helped delineate the function of proteins mutated in human NCL. In addition, NCL mutant mice have been tested for several different therapeutic approaches and as such they have become important pre-clinical models for validating treatment options. In this review we will assess the current state of mouse models of NCL with regards to their unique pathophysiology and how these mice have helped investigators achieve a better understanding of human NCL disease and therapy.

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.

The yeast Batten disease orthologue Btn1 controls endosome-Golgi retrograde transport via SNARE assembly

The human Batten disease gene CLN3 and yeast orthologue BTN1 encode proteins of unclear function. We show that the loss of BTN1 phenocopies that of BTN2, which encodes a retromer accessory protein involved in the retrieval of specific cargo from late endosomes (LEs) to the Golgi. However, Btn1 localizes to Golgi and regulates soluble N-ethyl-maleimide sensitive fusion protein attachment protein receptor (SNARE) function to control retrograde transport. Specifically, BTN1 overexpression and deletion have opposing effects on phosphorylation of the Sed5 target membrane SNARE, on Golgi SNARE assembly, and on Golgi integrity. Although Btn1 does not interact physically with SNAREs, it regulates Sed5 phosphorylation by modulating Yck3, a palmitoylated endosomal kinase. This may involve modification of the Yck3 lipid anchor, as substitution with a transmembrane domain suppresses the deletion of BTN1 and restores trafficking. Correspondingly, deletion of YCK3 mimics that of BTN1 or BTN2 with respect to LE-Golgi retrieval. Thus, Btn1 controls retrograde sorting by regulating SNARE phosphorylation and assembly, a process that may be adversely affected in Batten Disease patients.

Altered arginine metabolism in the central nervous system (CNS) of the Cln3-/- mouse model of juvenile Batten disease

BACKGROUND

Juvenile neuronal ceroid lipofuscinoses (JNCL) or juvenile Batten disease is a recessively inherited childhood neurodegenerative disorder resulting from a mutation in CLN3, which encodes a putative lysosomal protein of unknown function.

AIM

Recent evidence suggests that a disruption in CLN3 function results in altered regulation of arginine transport into lysosomes, and may influence intracellular arginine levels. We sought to investigate the possible consequences of arginine dysregulation in the brain of the Cln3(-/-) mouse model of JNCL.

METHODS

Using a combination of enzyme assays, metabolite profiling, quantitative reverse-transcription polymerase chain reaction and Western blotting, we analysed the activities and expression of enzymes involved in arginine metabolism in the cerebral cortex and cerebellum of Cln3(-/-) mice over several developmental time points.

RESULTS

We report subtle, but significant changes in the activities of enzymes involved in the citrulline-NO recycling pathway, and altered regulation of neuronal nitric oxide synthase in the cortex and cerebellum of Cln3(-/-) mice. In addition, a significant decrease in arginine transport into cerebellar granule cells was observed, despite an apparent upregulation of the cationic amino acid transporter-1 transporter at the cell surface. Our results provide further evidence that CLN3 function and arginine homeostasis are intricately related, and that cellular mechanisms may act to compensate for the loss of this protein.

CONCLUSIONS

This and other studies indicate that CLN3 dysfunction in JNCL may result in multiple disturbances in metabolism that together contribute to the pathophysiological processes underlying this disease.