- Research article
- Open Access
Membrane trafficking and mitochondrial abnormalities precede subunit c deposition in a cerebellar cell model of juvenile neuronal ceroid lipofuscinosis
© Fossale et al; licensee BioMed Central Ltd. 2004
Received: 04 October 2004
Accepted: 10 December 2004
Published: 10 December 2004
JNCL is a recessively inherited, childhood-onset neurodegenerative disease most-commonly caused by a ~1 kb CLN3 mutation. The resulting loss of battenin activity leads to deposition of mitochondrial ATP synthase, subunit c and a specific loss of CNS neurons. We previously generated Cln3Δex7/8 knock-in mice, which replicate the common JNCL mutation, express mutant battenin and display JNCL-like pathology.
To elucidate the consequences of the common JNCL mutation in neuronal cells, we used P4 knock-in mouse cerebella to establish conditionally immortalized CbCln3 wild-type, heterozygous, and homozygous neuronal precursor cell lines, which can be differentiated into MAP-2 and NeuN-positive, neuron-like cells. Homozygous CbCln3Δex7/8 precursor cells express low levels of mutant battenin and, when aged at confluency, accumulate ATPase subunit c. Recessive phenotypes are also observed at sub-confluent growth; cathepsin D transport and processing are altered, although enzyme activity is not significantly affected, lysosomal size and distribution are altered, and endocytosis is reduced. In addition, mitochondria are abnormally elongated, cellular ATP levels are decreased, and survival following oxidative stress is reduced.
These findings reveal that battenin is required for intracellular membrane trafficking and mitochondrial function. Moreover, these deficiencies are likely to be early events in the JNCL disease process and may particularly impact neuronal survival.
Juvenile neuronal ceroid lipofuscinosis (JNCL), or Batten disease, is a recessively inherited childhood-onset neurodegenerative disorder characterized by progressive blindness, seizures, motor and cognitive decline, and early death . The primary genetic defect (>80% disease chromosomes) leading to JNCL is a 1.02 kb genomic DNA deletion in the CLN3 gene, which eliminates exons 7 and 8 and surrounding intronic DNA, predicting a non-functional protein product .
The pathological hallmark of JNCL is autofluorescent ceroid lipofuscin deposits within autolysosomes that are enriched in subunit c of the mitochondrial ATP synthase complex [3–5]. Remarkably, these deposits are not only found in CNS neurons but are also abundant in non-neuronal cells outside of the nervous system. The relationship of subunit c deposits to the JNCL disease process, and the underlying reason for the neuronal specificity of the disease remain poorly understood.
The CLN3-encoded protein (battenin, also called CLN3 or cln3 p) is a highly conserved, ubiquitously expressed, multi-pass membrane protein  that localizes to the lysosome and other vesicular compartments [7–9]. Battenin function remains to be elucidated, although studies of btn1, the yeast CLN3 ortholog, have implicated battenin in lysosomal pH homeostasis and amino acid transport [10, 11].
To explore JNCL pathogenesis and battenin function, we previously generated a genetically precise JNCL mouse model. Cln3Δex7/8 knock-in mice harbor the ~1 kb common JNCL mutation and express a non-truncated mutant battenin isoform that is detectable with antibodies recognizing C-terminal epitopes. Homozygous Cln3Δex7/8 knock-in mice exhibit a progressive JNCL-like disease, with perinatal onset of subunit c deposition in many cell types and later onset of neuronal dysfunction and behavioral deficits . These findings suggest that the major JNCL defect leads to abnormal turnover of mitochondrial subunit c, in a manner that selectively compromises CNS neurons.
Currently, there is no suitable neuronal cell system to investigate the impact of the common JNCL mutation on biological processes. Therefore, we have established cerebellar neuronal precursor cell lines from Cln3Δex7/8 knock-in mice. Homozygous CbCln3Δex7/8 cells exhibit pathological hallmarks of the disease, and a survey of membrane organelles revealed membrane trafficking defects and mitochondrial dysfunction in homozygous mutant CbCln3Δex7/8 cells.
Generation of a genetically precise cerebellar JNCL cell model
Homozygous CbCln3Δex7/8cells express mutant battenin and display JNCL-like pathology
Homozygous CbCln3Δex7/8 cells and Cln3Δex7/8knock-in mice process cathepsin D abnormally
The impact of the altered cathepsin D processing on enzymatic activity was next tested to determine if altered enzymatic activity accounts for inefficient subunit c turnover. In a fluorogenic in vitro assay, cathepsin D activity in total cellular extracts was not significantly altered in homozygous CbCln3Δex7/8 cells (376 ± 89 RFU/μg total protein), versus wild-type cells (324 ± 58 RFU/μg total protein), although a consistent trend towards increased enzymatic activity in mutant cells was observed. Thus, cathepsin D transport and processing are disrupted in homozygous CbCln3Δex7/8 cells in a manner such that enzymatic activity appears to be relatively unaffected.
Homozygous CbCln3Δex7/8cells show abnormal membrane organelles
CbCln3Δex7/8 cerebellar precursor cells represent the first genetically accurate neuron-derived culture model of JNCL. Homozygous CbCln3Δex7/8 cells express mutant battenin and JNCL-hallmark mitochondrial ATPase subunit c accumulation, upon aging of cells at confluency. Moreover, this is the first study to indicate recessive endosomal/lysosomal membrane trafficking defects and mitochondrial dysfunction that precedes subunit c deposition in an accurate JNCL model. Importantly, these defects are likely to be early events in the JNCL disease process and may particularly impact neuronal function.
Abnormal cathepsin D localization and processing in homozygous CbCln3Δex7/8 cells and Cln3Δex7/8 mice likely reflects altered vesicular trafficking and/or lysosomal pH, which is known to impact cathepsin D processing [14, 16]. Indeed, CLN3 overexpression in HEK-293 cells altered lysosomal pH and cathepsin D processing , and lysosomal pH homeostasis is disrupted in JNCL [10, 15]. It is noteworthy that cathepsin B and the CLN2-encoded enzyme, TPPI, are also altered in JNCL [18–20]. Nevertheless, despite the cathepsin D protein alterations that are observed in homozygous CbCln3Δex7/8 cells, cathepsin D enzymatic activity does not appear to be reduced. Thus, decreased cathepsin D activity is unlikely to account for subunit c accumulation in JNCL.
Aging of homozygous CbCln3Δex7/8 cells at confluency is necessary to induce significantly accumulated subunit c protein. However, membrane organelle disruptions precede subunit c accumulation in homozygous CbCln3Δex7/8 cells, since they are observed without aging at confluency. Lysosomal and endosomal size and distribution are altered, and mitochondria are abnormally elongated and functionally compromised in sub-confluent homozygous CbCln3Δex7/8 cultures. These observations argue that membrane trafficking defects do not result from excessive subunit c accumulation compromising the lysosome, but rather are early events in the disease process preceding subunit c accumulation. Mitochondrial abnormalities, which have also been reported in JNCL patients and other animal models [21–23], may result from ineffective turnover by autophagy, a lysosomal-targeted pathway . Alternatively, or simultaneously, battenin deficiency may impact mitochondrial function upstream of turnover, affecting mitochondrial biogenesis and/or altered transport and processing of mitochondrial proteins.
In wild-type CbCln3 neuronal precursor cells battenin primarily co-localizes with early and late endosomes. Battenin immunostaining in homozygous CbCln3Δex7/8 neuronal precursors is significantly reduced in abundance, but mutant signal also co-localizes with endosomal markers suggesting mutant battenin protein with C-terminal epitopes is trafficked similar to wild-type protein. In other studies, CLN3/battenin protein localization has been reported to partially overlap with late endosomes and lysosomes in non-neuronal cells , and to lysosomes, synaptosomes  and endosomes [9, 25] in neurons. These data jointly indicate that battenin resides in a subset of vesicular compartments linking multiple membrane trafficking pathways, perhaps functioning in vesicular transport and/or fusion. Endocytic and lysosomal-targeted pathways, including mitochondrial autophagy, are specifically implicated in this study.
The membrane trafficking and mitochondrial deficits uncovered in homozygous CbCln3Δex7/8 cells are likely to particularly impact neuronal function. Neurotransmission heavily relies on membrane vesicle transport, and a high-energy metabolism may further sensitize neurons to the loss of battenin activity. Thus, our panel of wild-type, heterozygous, and homozygous CbCln3Δex7/8 cerebellar cells provide an ideal model system to further elucidate battenin function and JNCL pathogenesis.
Antibody and cell staining reagents
Nestin (clone Rat 401, 2 μg/ml), Lamp 1 (clone 1D4B, 6 μg/ml), and Lamp 2 (clone Abl-93; 6 μg/ml) monoclonal antibodies were obtained from the Developmental Studies Hybridoma Bank, maintained by The University of Iowa, Department of Biological Sciences. Batp1 (1 μg/ml) was previously described . Anti-subunit c antibody (0.7 μg/ml) was kindly provided by Dr. Kominami (Juntendo University, Tokyo, Japan). Additional antibodies were as follows: GFAP, 1:2000 (DAKO Corporation); calbindin, 1:5000 (Sigma); NeuN, 10 μg/ml (Chemicon); SV40 T antigen (Pab 101), 2 μg/ml (Santa Cruz Biotechnology); cathepsin D (C-20), 2 μg/ml (Santa Cruz Biotechnology); cytochrome c oxidase subunit IV (cox4), 1:1000 (BD Biosciences Clontech); PDI (H-160), 2 μg/ml (Santa Cruz Biotechnology); GM130, 1 μg/ml (BD Transduction Labs); α-tubulin, 1:15,000 (Sigma); grp75, 1:200 (Stressgen); early endosome antigen-1 (EEA1), 2 μg/ml (Santa Cruz Biotechnology); rab 7, 4 μg/ml (Santa Cruz Biotechnology). All fluorescent secondary antibodies were obtained from Jackson ImmunoResearch Laboratories and HRP-conjugated secondary antibodies were obtained from Amersham Biosciences. Additional cell markers were as follows: VVL-biotin, 1:2000 (Vector Laboratories), 10,000 MW dextran-FITC, 1 mg/ml and Lysotracker Red DND-99, 500 nM (Molecular Probes).
Generation, maintenance and differentiation of CbCln3cerebellar neuronal precursor lines
Cln3Δex7/8 knock-in mice have been previously described . Littermate pups from heterozygote crosses were used for primary culture establishment, by previously described methods that yield cerebellar granule neuron-enriched cultures . Postnatal day 4 (P4) cerebella were dissected in Hank's Balanced Salt Solution (HBSS, Sigma), supplemented with 35 mM glucose. Tail biopsies were also collected for genomic DNA isolation and genotypic analysis. Trypsin/EDTA (10 mg/ml, Sigma) and DNase I (100 μg/ml, Sigma), suspended in HBSS, helped dissociate cerebella for primary culture plating onto 0.01% poly-ornithine (Sigma) coated 100 mm dishes. Primary cultures from individual cerebella were cultured overnight at 37°C, 5% CO2, in granule neuron growth media (Dulbecco's Modified Eagle Medium [DMEM, Gibco BRL #11995-065], 10% fetal bovine serum [Sigma #F-2442], supplemented with 24 mM KCl). Infection was performed the following day with defective retrovirus transducing the temperature-sensitive tsA58/U19 large T antigen and a selection neomycin-resistance cassette , as previously described . Following infection, cultures were shifted to the tsA58/U19 permissive growth temperature of 33°C and selection was in the same growth media as above, with 200 μg/ml G418. Conditionally immortalized colonies were isolated 3–9 weeks post-infection and expanded for frozen stocks and further sub-clone isolation. Multiple clonal lines were obtained for each genotype and all phenotypes were confirmed in at least 2 independent CbCln3 cell lines. CbCln3 cell lines were maintained on 0.01% poly-ornithine coated dishes at 30–90% confluency, in 33°C and 5% CO2 atmosphere. Passage number was recorded (up to ~20 passages), but had no apparent impact on phenotype. Neuronal differentiation was as previously described  with the following cocktail: 10 ng/ml α-FGF, 250 μM IBMX, 200 nM TPA, 50 μM forskolin, 5 μM dopamine (Sigma).
Genotyping and RT-PCR
Genomic DNA was extracted from tail biopsies and cell pellets as described (Cotman et al., 2002). Cln3Δex7/8 knock-in allele PCR genotyping was with wild-type primers, WtF (5'-CAGCATCTCCTCAGGGCTA-3') and WtR (5'-CCAACATAGAAAGTAGGGTGTGC-3') to yield a ~250 bp band and knock-in primers, 552F (5'-GAGCTTTGTTCTGGTTGCCTTC-3') and Ex9RA (5'-GCAGTCTCTGCCTCGTTTTCT-3') to yield a ~500 bp band. PCR cycling conditions were 95°C for 30 seconds, 58°C for 30 seconds, and 72°C for 35 seconds, repeated for 34 cycles. Total RNA isolation and Cln3 RT-PCR primers and procedures have been previously described .
Subunit c accumulation assay
Cells were seeded into 4-well chamber-slides (Falcon) at a density of 5 × 104 cells per well for microscopy studies, or into 100 mm dishes (Falcon) at a density of 5 × 105 cells per dish for protein extraction. Cells were typically >95% confluent one day post-plating, and the following day was considered 1-day post-confluency. At the indicated times, cells were either fixed with 4% formaldehyde in phosphate buffered saline (PBS), pH 7.4, for 20 minutes and processed for autofluorescence/subunit c immunostaining, or cell pellets were collected for total protein extraction.
Alternatively, cells were fixed with 2.5% glutaraldehyde/2% paraformaldehyde in 0.1 M cacodylate buffer, pH 7.4 for 1 hour and subsequently post-fixed and processed for TEM analysis as described . In confocal microscopy studies, autofluorescent signal was observed over multiple wavelengths. For co-staining, settings were reduced such that autofluorescent signal did not contribute to antibody label signal.
Immunostaining and Immunoblot analysis
For immunostaining, cells were seeded at a density of 3–5 × 104 cells per well in 4-well chamber-slides and grown overnight at 33°C, unless indicated otherwise. Fixation was with ice-cold 4% formaldehyde in PBS, pH 7.4, for 20 minutes, or with ice-cold methanol/acetone (1:1) for 10 minutes at -20°C followed by air-drying (antibody-dependent). Cells were washed with PBS at least 2 times, 5 minutes per wash, between each of the following steps of the staining procedure: 0.1 M glycine in PBS for 5 minutes, 0.05% or 0.1% (antibody-dependent) Triton X-100 (Fisher Scientific) in PBS for 5 minutes, 2% bovine serum albumin (BSA) in PBS for 30 minutes, primary antibody diluted in 2% BSA/PBS for 90 minutes, secondary antibody diluted in 2% BSA/PBS for 60 minutes. All incubations were carried out at room temperature. Following staining procedures, slides were coverslipped with Vectashield mounting medium (Vector Laboratories) and analyzed on a BioRad Radiance 2100 confocal microscope (Biorad), with identical exposure settings for wild-type and mutant like images. All comparisons of wild-type and mutant staining were performed in Adobe Photoshop with identical brightness and contrast adjustments.
Total proteins were isolated from cell pellets by extraction with ice-cold 20 mM Tris, pH 7.4, 1% Triton X-100 (membrane-research grade, Roche), plus protease inhibitors (Complete mini tablet, 0.7 μg/ml pepstatin A, 2 μg/ml aprotinin, 5 μg/ml leupeptin [Roche]). Following homogenization through a 25-gauge needle (~10 passes), extracts were centrifuged at 1000 × g for 10 minutes, at 4°C, to remove debris. Typically, 20–40 μg of protein (determined by Bio-rad Dc Protein Assay) was separated by SDS-PAGE, for subsequent immunoblotting, as described . 16.5% tris-tricine SDS-PAGE gels were used for subunit c immunoblotting experiments.
Cathepsin D activity assay
100 mm tissue culture dishes, which were approximately 80–90% confluent, were washed briefly with ice-cold PBS, and total protein extracts were isolated by scraping cells into 10 mM Tris, pH 7.4, 0.1% Triton X-100 followed by incubation on ice, for 20 minutes. The insoluble material was centrifuged at 14,000 g, the supernatant was isolated, and protein concentration was determined using the Bio-rad Dc Protein Assay. 50–70 μg of total protein extract were used to measure cathepsin D activity using the Fluorogenic Innozyme™ Cathepsin D Immunocapture Activity Assay Kit (EMD Biosciences) according to the manufacturer's recommendations. Relative fluorescence was measured using an Analyst AD plate reader (Molecular Devices) with the following filters and settings: excitation filter, 360-35; emission filter, 400-20, Flash lamp with 100 readings/well, 100 ms between readings, and 100,000 μs integration time.
Lysosomal staining and endocytic uptake
Cells were seeded at a density of 3–5 × 104 cells per well in 4-well chamber-slides and grown overnight at 33°C. Growth media was exchanged for fresh, pre-warmed growth media containing 500 nM Lysotracker or 1 mg/ml dextran-FITC, and cells were incubated at 33°C for 45 minutes or 15 minutes, respectively. Following labeling, cells were immediately placed on ice and washed for 10 minutes in ice-cold dye-free media, and fixed with 4% formaldehyde in PBS, for 20 minutes on ice. Chambers were removed and slides were coverslipped with Vectashield mounting media for confocal microscopy analysis, as described above.
Morphometric analysis of mitochondria
TEM photomicrographs (10,000 × – 40,000 × magnification) were taken from random grid fields. For length measurements, the longest side of each mitochondria was measured in centimeters, and along the length of the mitochondria, width measurements were taken every 2.5–4 mm (dependent on the magnification of the micrograph image). Following measurement, all numbers were normalized to reflect one magnification and data was analyzed using Microsoft Excel software. To ascertain unmagnified mitochondrial size, final measurement data, in centimeters, was converted to nanometers according to scale bar representation.
ATP was measured by using the CellTiter-GLO® Luminescent Cell Viability kit (Promega), according to the manufacturer's recommendations. Briefly, cells were plated in a black opaque-walled 96 well plate (Packard Bioscience) at a density of 20000/well and incubated at 33°C overnight. The following day, CellTiter-GLO® Reagent was added to each well and cell lysis was induced by mixing 2 minutes. An ATP standard curve was prepared in the same plate. Before recording luminescence with a microplate luminometer (MicroLumat Plus LB 96V, Berthold Techonologies), the plate was dark adapted for 10 minutes at room temperature to stabilize the luminescence signal.
Hydrogen peroxide treatment assay
Cells were plated at a density of 10,000 cells/well in 96-well plates and incubated at 33°C overnight. The following day, fresh media containing varying concentrations of hydrogen peroxide was dispersed to each well. Cells were incubated in the presence of hydrogen peroxide for 24 hours, at 33°C, and viability was measured using the CellTiter-96® AQueous Non-Radioactive Cell Proliferation Assay (Promega), according to the manufacturer's specifications.
The authors thank Dr. E. Kominami for antibody to subunit c of mitochondrial ATPase, L. Trakimas and M. Ericsson of the Harvard Medical School Electron Microscope Facility, Dr. David Sulzer for assistance with TEM analysis, Dr. Sylvie Breton for assistance with endocytosis experiments, and Dr. Andre Bernards and James Follen for assistance with the cathepsin D activity assay. This work was supported by NIH/NINDS grant # NS 33648. Dr. S.L. Cotman received fellowship funding from the Batten Disease Support and Research Association (BDSRA).
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