pH-dependent localization of Btn1p in the yeast model for 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.

in vivo localization of the neuronal ceroid lipofuscinosis proteins, CLN3 and CLN7, at endogenous expression levels

The neuronal ceroid lipofuscinoses are a group of recessively inherited, childhood-onset neurodegenerative conditions. Several forms are caused by mutations in genes encoding putative lysosomal membrane proteins. Studies of the cell biology underpinning these disorders are hampered by the poor antigenicity of the membrane proteins, which makes visualization of the endogenous proteins difficult. We have used Drosophila to generate knock-in YFP-fusions for two of the NCL membrane proteins: CLN7 and CLN3. The YFP-fusions are expressed at endogenous levels and the proteins can be visualized live without the need for overexpression. Unexpectedly, both CLN7 and CLN3 have restricted expression in the CNS of Drosophila larva and are predominantly expressed in the glia that form the insect blood-brain-barrier. CLN7 is also expressed in neurons in the developing visual system. Analogous with murine CLN3, Drosophila CLN3 is strongly expressed in the excretory and osmoregulatory Malpighian tubules, but the knock-in also reveals unexpected localization of the protein to the apical domain adjacent to the lumen. In addition, some CLN3 protein in the tubules is localized within mitochondria. Our in vivo imaging of CLN7 and CLN3 suggests new possibilities for function and promotes new ideas about the cell biology of the NCLs.

The neuronal ceroid lipofuscinoses: Opportunities from model systems

The neuronal ceroid lipofuscinoses are a group of severe and progressive neurodegenerative disorders, generally with childhood onset. Despite the fact that these diseases remain fatal, significant breakthroughs have been made in our understanding of the genetics that underpin these conditions. This understanding has allowed the development of a broad range of models to study disease processes, and to develop new therapeutic approaches. Such models have contributed significantly to our knowledge of these conditions. In this review we will focus on the advantages of each individual model, describe some of the contributions the models have made to our understanding of the broader disease biology and highlight new techniques and approaches relevant to the study and potential treatment of the neuronal ceroid lipofuscinoses. This article is part of a Special Issue entitled: "Current Research on the Neuronal Ceroid Lipofuscinoses (Batten Disease)".

Evidence for aberrant astrocyte hemichannel activity in Juvenile Neuronal Ceroid Lipofuscinosis (JNCL)

Juvenile Neuronal Ceroid Lipofuscinosis (JNCL) is a lysosomal storage disease caused by an autosomal recessive mutation in CLN3 that leads to vision loss, progressive cognitive and motor decline, and premature death. Morphological evidence of astrocyte activation occurs early in the disease process and coincides with regions where neuronal loss eventually ensues. However, the consequences of CLN3 mutation on astrocyte function remain relatively ill-defined. Astrocytes play a critical role in CNS homeostasis, in part, by their ability to regulate the extracellular milieu via the formation of extensive syncytial networks coupled by gap junction (GJ) channels. In contrast, unopposed hemichannels (HCs) have been implicated in CNS pathology by allowing the non-discriminant passage of molecules between the intracellular and extracellular milieus. Here we examined acute brain slices from CLN3 mutant mice (CLN3^Δex7/8) to determine whether CLN3 loss alters the balance of GJ and HC activity. CLN3^Δex7/8 mice displayed transient increases in astrocyte HC opening at postnatal day 30 in numerous brain regions, compared to wild type (WT) animals; however, HC activity steadily decreased at postnatal days 60 and 90 in CLN3^Δex7/8 astrocytes to reach levels lower than WT cells. This suggested a progressive decline in astrocyte function, which was supported by significant reductions in glutamine synthetase, GLAST, and connexin expression in CLN3^Δex7/8 mice compared to WT animals. Based on the early increase in astrocyte HC activity, CLN3^Δex7/8 mice were treated with the novel carbenoxolone derivative INI-0602 to inhibit HCs. Administration of INI-0602 for a one month period significantly reduced lysosomal ceroid inclusions in the brains of CLN3^Δex7/8 mice compared to WT animals, which coincided with significant increases in astrocyte GJ communication and normalization of astrocyte resting membrane potential to WT levels. Collectively, these findings suggest that alterations in astrocyte communication may impact the progression of JNCL and could offer a potential therapeutic target.

Osmotic stress changes the expression and subcellular localization of the Batten disease protein CLN3

Juvenile CLN3 disease (formerly known as juvenile neuronal ceroid lipofuscinosis) is a fatal childhood neurodegenerative disorder caused by mutations in the CLN3 gene. CLN3 encodes a putative lysosomal transmembrane protein with unknown function. Previous cell culture studies using CLN3-overexpressing vectors and/or anti-CLN3 antibodies with questionable specificity have also localized CLN3 in cellular structures other than lysosomes. Osmoregulation of the mouse Cln3 mRNA level in kidney cells was recently reported. To clarify the subcellular localization of the CLN3 protein and to investigate if human CLN3 expression and localization is affected by osmotic changes we generated a stably transfected BHK (baby hamster kidney) cell line that expresses a moderate level of myc-tagged human CLN3 under the control of the human ubiquitin C promoter. Hyperosmolarity (800 mOsm), achieved by either NaCl/urea or sucrose, dramatically increased the mRNA and protein levels of CLN3 as determined by quantitative real-time PCR and Western blotting. Under isotonic conditions (300 mOsm), human CLN3 was found in a punctate vesicular pattern surrounding the nucleus with prominent Golgi and lysosomal localizations. CLN3-positive early endosomes, late endosomes and cholesterol/sphingolipid-enriched plasma membrane microdomain caveolae were also observed. Increasing the osmolarity of the culture medium to 800 mOsm extended CLN3 distribution away from the perinuclear region and enhanced the lysosomal localization of CLN3. Our results reveal that CLN3 has multiple subcellular localizations within the cell, which, together with its expression, prominently change following osmotic stress. These data suggest that CLN3 is involved in the response and adaptation to cellular stress.

Use of model organisms for the study of neuronal ceroid lipofuscinosis

Neuronal ceroid lipofuscinoses are a group of fatal progressive neurodegenerative diseases predominantly affecting children. Identification of mutations that cause neuronal ceroid lipofuscinosis, and subsequent functional and pathological studies of the affected genes, underpins efforts to investigate disease mechanisms and identify and test potential therapeutic strategies. These functional studies and pre-clinical trials necessitate the use of model organisms in addition to cell and tissue culture models as they enable the study of protein function within a complex organ such as the brain and the testing of therapies on a whole organism. To this end, a large number of disease models and genetic tools have been identified or created in a variety of model organisms. In this review, we will discuss the ethical issues associated with experiments using model organisms, the factors underlying the choice of model organism, the disease models and genetic tools available, and the contributions of those disease models and tools to neuronal ceroid lipofuscinosis research. This article is part of a Special Issue entitled: The Neuronal Ceroid Lipofuscinoses or Batten Disease.

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.

BTN1, the Saccharomyces cerevisiae homolog to the human Batten disease gene, is involved in phospholipid distribution

BTN1, the yeast homolog to human CLN3 (which is defective in Batten disease), has been implicated in the regulation of vacuolar pH, potentially by modulating vacuolar-type H⁺-ATPase (V-ATPase) activity. However, we report that Btn1p and the V-ATPase complex do not physically interact, suggesting that any influence that Btn1p has on V-ATPase is indirect. Because membrane lipid environment plays a crucial role in the activity and function of membrane proteins, we investigated whether cells lacking BTN1 have altered membrane phospholipid content. Deletion of BTN1 (btn1-Δ) led to a decreased level of phosphatidylethanolamine (PtdEtn) in both mitochondrial and vacuolar membranes. In yeast there are two phosphatidylserine (PtdSer) decarboxylases, Psd1p and Psd2p, and these proteins are responsible for the synthesis of PtdEtn in mitochondria and Golgi-endosome, respectively. Deletion of both BTN1 and PSD1 (btn1-Δ psd1-Δ) led to a further decrease in levels of PtdEtn in ER membranes associated to mitochondria (MAMs), with a parallel increase in PtdSer. Fluorescent-labeled PtdSer (NBD-PtdSer) transport assays demonstrated that transport of NBD-PtdSer from the ER to both mitochondria and endosomes and/or vacuole is affected in btn1-Δ cells. Moreover, btn1-Δ affects the synthesis of PtdEtn by the Kennedy pathway and impairs the ability of psd1-Δ cells to restore PtdEtn to normal levels in mitochondria and vacuoles by ethanolamine addition. In summary, lack of Btn1p alters phospholipid levels and might play a role in regulating their subcellular distribution.

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.