Batten disease and mitochondrial pathways of proteolysis

Photoreceptor phagosome processing defects and disturbed autophagy in retinal pigment epithelium of Cln3Δex1-6 mice modelling juvenile neuronal ceroid lipofuscinosis (Batten disease)

Retinal degeneration and visual impairment are the first signs of juvenile neuronal ceroid lipofuscinosis caused by CLN3 mutations, followed by inevitable progression to blindness. We investigated retinal degeneration in Cln3(Δex1-6) null mice, revealing classic 'fingerprint' lysosomal storage in the retinal pigment epithelium (RPE), replicating the human disease. The lysosomes contain mitochondrial F0-ATP synthase subunit c along with undigested membranes, indicating a reduced degradative capacity. Mature autophagosomes and basal phagolysosomes, the terminal degradative compartments of autophagy and phagocytosis, are also increased in Cln3(Δex1) (-6) RPE, reflecting disruption to these key pathways that underpin the daily phagocytic turnover of photoreceptor outer segments (POS) required for maintenance of vision. The accumulated autophagosomes have post-lysosome fusion morphology, with undigested internal contents visible, while accumulated phagosomes are frequently docked to cathepsin D-positive lysosomes, without mixing of phagosomal and lysosomal contents. This suggests lysosome-processing defects affect both autophagy and phagocytosis, supported by evidence that phagosomes induced in Cln3(Δex1) (-) (6)-derived mouse embryonic fibroblasts have visibly disorganized membranes, unprocessed internal vesicles and membrane contents, in addition to reduced LAMP1 membrane recruitment. We propose that defective lysosomes in Cln3(Δex1) (-) (6) RPE have a reduced degradative capacity that impairs the final steps of the intimately connected autophagic and phagocytic pathways that are responsible for degradation of POS. A build-up of degradative organellar by-products and decreased recycling of cellular materials is likely to disrupt processes vital to maintenance of vision by the RPE.

Time-course of hormonal induction of mitochondrial glycerophosphate dehydrogenase biogenesis in rat liver

Thyroid hormones are important regulators of mitochondrial metabolism. Due to their complex mechanism of action, the timescale of different responses varies from minutes to days. In this work, we studied selective T3 induction of the inner mitochondrial membrane enzyme-glycerophosphate dehydrogenase (mGPDH) in liver of euthyroid rats. We correlated the kinetics of the T3 level in blood, the mRNA level in liver, the activity and amount of mGPDH in liver mitochondria after a single dose of T3. The T3 level reached maximum after 1 h (80 nmol/l) and subsequently rapidly decreased. mGPDH mRNA increased also relatively fast, reaching a maximum after 12 h and fell to the control level after 72 h. An increase of mGPDH activity could be already found after 6 h and reached a maximum after 24 h in accordance with the increase in mGPDH content (2.4-fold vs. 2.7-fold induction). After 72 h, the mGPDH activity showed a significant 30% decrease. When the rats received three subsequent doses of T3, the increase of mGPDH activity was 2-fold higher than after a single T3 dose. The results demonstrate that mGPDH displays rapid induction as well as decay upon disappearance of a hormonal stimulus, indicating a rather short half-life of this inner mitochondrial membrane enzyme.

Protein degradation in mitochondria: implications for oxidative stress, aging and disease: a novel etiological classification of mitochondrial proteolytic disorders

The mitochondrial genome encodes just a small number of subunits of the respiratory chain. All the other mitochondrial proteins are encoded in the nucleus and produced in the cytosol. Various enzymes participate in the activation and intramitochondrial transport of imported proteins. To finally take their place in the various mitochondrial compartments, the targeting signals of imported proteins have to be cleaved by mitochondrial processing peptidases. Mitochondria must also be able to eliminate peptides that are internally synthesized in excess, as well as those that are improperly assembled, and those with abnormal conformation caused by mutation or oxidative damage. Damaged mitochondrial proteins can be removed in two ways: either through lysosomal autophagy, that can account for at most 25-30% of the biochemically estimated rates of average mitochondrial catabolism; or through an intramitochondrial proteinolytic pathway. Mitochondrial proteases have been extensively studied in yeast, but evidence in recent years has demonstrated the existence of similar systems in mammalian cells, and has pointed to the possible importance of mitochondrial proteolytic enzymes in human diseases and ageing. A number of mitochondrial diseases have been identified whose mechanisms involve proteolytic dysfunction. Similar mechanisms probably play a role in diminished resistance to oxidative stress, and in the aging process. In this paper we review current knowledge of mammalian mitochondrial proteolysis, under normal conditions and in several disease states, and we propose an etiological classification of human diseases characterized by a decline or loss of function of mitochondrial proteolytic enzymes.

Submitochondrial distribution and delayed proteolysis of subunit c of the H+-transporting ATP-synthase in ovine ceroid-lipofuscinosis

The neuronal ceroid-lipofuscinose (NCL) are recessively inherited lysosomal storage diseases in children and animals. The major stored protein in many of these diseases is subunit c of the mitochondrial inner membrane H+-transporting ATP-synthase. Previous studies of naturally occurring ovine ceroid-lipofuscinosis (OCL) in South Hampshire sheep showed that the genes and transcripts for subunit c were normal and inferred that this protein was expressed normally in mitochondria prior to storage in lysosomes. Accumulation in mitochondria has not been conclusively established and we have therefore used the South Hampshire model to demonstrate approximately 1.8-fold normal levels of subunit c in mitochondrial inner membranes prepared from liver. Other mitochondrial inner membrane and ATP-synthase proteins that could be detected by mass spectrometry (MS) or two-dimensional electrophoresis (2-DE) were present in normal amounts. The accumulating subunit c showed normal post-translational modification but was abnormally resistant to proteolysis. These results are consistent with the hypothesis that OCL may result from a mitochondrial disorder that affects turnover of correctly expressed subunit c, although we cannot exclude the possibility that a postmitochondrial defect delays processing of subunit c out of mitochondria.

Postmortem changes in the level of brain proteins

A systematic study on postmortem changes of brain proteins has not been performed so far and information is limited to basic principles of specific or nonspecific proteolysis or proteolysis of individual proteins. We studied protein level alterations in rat brain of animals kept at 23 degrees C for several postmortem times up to 72 h. Brain tissue protein extracts were analyzed by two-dimensional electrophoresis and the proteins with different levels were identified by matrix-assisted laser desorption ionization mass spectrometry. The changes observed mainly concerned structural proteins and enzymes. The levels of dihydropyrimidinase-related protein-2 decreased within 6 h and two new spots were detected representing shorter forms of the protein. Most of the other alterations appeared about 48 h postmortem. The most significant were reduced levels of neurofilament, alpha-internexin, synaptosomal-associated protein 25, glial fibrillary acidic protein, heat shock proteins, and dynamin-1; increased levels of 14-3-3 proteins and spectrin; and generation of shorter forms of certain proteins, such as tubulins, actin, and serum albumin. The results may be useful in neuropathology and brain protein studies.

Juvenile neuronal ceroid lipofuscinosis

A case of juvenile neuronal ceroid lipofuscinosis (JNCL) diagnosed on the basis of clinical features, electrophysiologic studies and skin electron microscopy is reported. JNCL was suspected on the basis of characteristic symptoms including progressive loss of vision, seizures, mental retardation and motor disabilities. Diagnosis was confirmed by neurophysiological and biopsy studies. The disease is caused by 23 different mutations in a gene recently isolated on chromosome 16 p11.2-12.1. Although universally fatal, characterisation of mutations can help in prenatal diagnosis in future pregnancies.

A novel endoproteolytic processing activity in mitochondria of erythroid cells and the role in heme synthesis

The erythroid isoform of aminolevulinate synthase (eALAS) protein is a major control point in erythroid heme synthesis and hemoglobin formation. Erythroid cells were extracted from mouse blood and bone marrow and metabolically labeled with 35S-methionine. This was followed by immunoprecipitation of eALAS protein products. The results show that the N-terminus of the expected full-length 59-kd form of the eALAS protein is truncated in bone marrow erythroid cells by approximately 7 kd. More differentiated erythroid cells in the peripheral blood exhibit very little of this protein truncation. Erythroid cells from the bone marrow were isolated using monoclonal antibody TER-119 and were shown to contain a unique endoprotease activity that could cleave the eALAS protein to the shorter form in vitro. With or without the mitochondrial signal sequence, the eALAS protein could serve as a substrate for the cleavage. This cleavage renders a functional eALAS protein and only removes a domain of unclear function, which has previously been reported to vary in size as a result of alternative RNA splicing. The protease activity was enriched from the membranes of mitochondria from bone marrow cells and was shown to be different from mitochondrial processing peptidase, medullasin, and other known proteases. Apart from the mitochondrial processing peptidase that cleaves the import signal sequence, this is the first description of a mitochondrially located site-specific processing protease activity.

A novel endoproteolytic processing activity in mitochondria of erythroid cells and the role in heme synthesis

The erythroid isoform of aminolevulinate synthase (eALAS) protein is a major control point in erythroid heme synthesis and hemoglobin formation. Erythroid cells were extracted from mouse blood and bone marrow and metabolically labeled with 35S-methionine. This was followed by immunoprecipitation of eALAS protein products. The results show that the N-terminus of the expected full-length 59-kd form of the eALAS protein is truncated in bone marrow erythroid cells by approximately 7 kd. More differentiated erythroid cells in the peripheral blood exhibit very little of this protein truncation. Erythroid cells from the bone marrow were isolated using monoclonal antibody TER-119 and were shown to contain a unique endoprotease activity that could cleave the eALAS protein to the shorter form in vitro. With or without the mitochondrial signal sequence, the eALAS protein could serve as a substrate for the cleavage. This cleavage renders a functional eALAS protein and only removes a domain of unclear function, which has previously been reported to vary in size as a result of alternative RNA splicing. The protease activity was enriched from the membranes of mitochondria from bone marrow cells and was shown to be different from mitochondrial processing peptidase, medullasin, and other known proteases. Apart from the mitochondrial processing peptidase that cleaves the import signal sequence, this is the first description of a mitochondrially located site-specific processing protease activity.

Batten's disease: clues to neuronal protein catabolism in lysosomes

Neuronal ceroid lipofuscinosis (Batten disease) encompasses a group of 8 or more inherited lysosomal storage diseases, with an overall frequency of 1 in 12,500 births. All are characterized by progressive blindness and dementia and were initially classified on the basis of age of onset, clinical phenotype and ultrastructural characterization of the storage material as granular osmiophilic deposits, curvilinear bodies or fingerprint bodies. Recent research has shown that the various forms of Batten disease result from mutations in at least 8 genes which code for proteins involved in different aspects of lysosomal protein catabolism. These include palmitoyl:protein thioesterase 1 (CLN1), tripeptidylpeptidase 1 (CLN2), cathepsin D (CLN8), and two membrane proteins of unknown function (CLN3 and CLN5). Biochemically, Batten disease is characterized by the accumulation in neurons and other cells of an autofluorescent pigment which has resisted many attempts at analysis. In this review we attempt to relate our current understanding of the nature of the storage material in Batten disease with this genetic information. We conclude that the 8 genes probably code for proteins which facilitate the degradation of post-translationally modified proteins in lysosomes, suggesting that the turnover of these proteins is highest in cortical neurons.

Intracellular trafficking of the JNCL protein CLN3

Juvenile neuronal ceroid lipofuscinosis is a lysosomal storage disease that causes visual impairment, progressive mental deterioration, and eventually death. A predominant 1.02-kb deletion as well as other mutations have been described in the CLN3 gene. Lacking significant identity with proteins of known function and no overt targeting signals within the primary amino acid sequence, accurate predictions of the intracellular location and function could not be made. Further, recent conflicting reports identified CLN3 as either a lysosomal or a mitochondrial protein. Transfection experiments using native and epitope-tagged fusion proteins were evaluated to help delineate CLN3 localization. We confirmed by immunohistochemistry and brefeldin A treatment that NH2-terminal green fluorescence protein (GFP)-CLN3 fusion proteins were retained in the Golgi apparatus, with no colocalization with mitochondrial markers. Anti-CLN3 antibodies directed against amino acids 67-90 of CLN3 were generated and shown to be specific for a 50-kDa protein in HEK 293 cells and GFP-CLN3 in transfected cells. However, cells transfected with nontagged CLN3 or carboxyl-terminal-tagged CLN3 were not immunoreactive with anti-CLN3 antibodies, suggesting that normally, the amino terminus interacts with other molecules. Thus, tags on the NH2-terminus probably inhibited these interactions and movement of CLN3 from the Golgi to more distal compartments. Also, CLN3 tagged at the COOH-terminus with either GFP or FLAG epitopes were retained in the ER, indicating a role for the COOH-terminus in trafficking. Taken together, these data confirm that CLN3 traffics through the ER and Golgi.

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.

Rate of accumulation of Luxol Fast Blue staining material and mitochondrial ATP synthase subunit 9 in motor neuron degeneration mice

The rate of accumulation of Luxol Fast Blue staining material in the hippocampus of motor neuron degeneration (mnd/mnd) mice, a model of Batten Disease, was quantitated. Stained material increased linearly up to 8 months of age. A quantitative immunoassay was used to measure levels of mitochondrial ATP synthase subunit 9 in brain and liver of mnd/mnd mice. Levels of subunit 9 increased progressively throughout the lifespan of mnd/mnd mice reaching levels approximately 5-fold higher than in control animals. The rate of accumulation of subunit 9 is not consistent with any simple complete or partial degradation defect that is constant throughout the animal's life. Two more complicated models are discussed which are consistent with the observed accumulation rate of subunit 9.

Turnover of F1F0-ATP synthase subunit 9 and other proteolipids in normal and Batten disease fibroblasts

Fibroblasts derived from patients with late infantile neuronal ceroid lipofucsinosis (NCL) and from a mouse model of NCL are similar to cells in intact animals in that they accumulate subunit 9 of mitochondrial F1F0-ATP synthase (F-ATPase) (Tanner, A., Dice, J.F., Cell Biol. Int. 19 (1995) 71-75). We now report no differences in the synthetic rates of F-ATPase subunit 9 in such affected cells when compared to control cells. However, the degradation rates of F-ATPase subunit 9 are reduced in both the affected human and mouse cells. This reduced degradation applies only to subunit 9 and the homologous vacuolar ATPase subunit among five distinct, reproducible proteolipid bands analyzed. Approximately 15% of newly synthesized F-ATPase subunit 9 is rapidly degraded in control cells, but this rapidly degraded component is absent in both the human and mouse NCL fibroblasts. At confluence, when the accumulated F-ATPase subunit 9 transiently disappears from human NCL fibroblasts, there is an increased degradation of all proteolipids. The pathway of degradation that is enhanced at confluence is likely to correspond to lysosomal macroautophagy. We confirmed that lysosomes were able to degrade F-ATPase subunit 9 after endocytosis of radiolabeled mitochondria. Human NCL fibroblasts were less active than control cells in this lysosomal degradation of endocytosed F-ATPase subunit 9. However, this difference was not specific for F-ATPase subunit 9 since it also applied to total endocytosed mitochondrial protein. We conclude that degradation of F-ATPase subunit 9 can occur by multiple pathways and that a mitochondrial pathway of proteolysis is defective in the late infantile human and mouse forms of NCL.

Impaired folding and subunit assembly as disease mechanism: the example of medium-chain acyl-CoA dehydrogenase deficiency

Rapid progress in DNA technology has entailed the possibility of readily detecting mutations in disease genes. In contrast to this, techniques to characterize the effects of mutations are still very time consuming. It has turned out that many of the mutations detected in disease genes are missense mutations. Characterization of the effect of these mutations is particularly important in order to establish that they are disease causing and to estimate their severity. We use the experiences with investigation of medium-chain acyl-CoA dehydrogenase deficiency as an example to illustrate that (i) impaired folding is a common effect of missense mutations occurring in genetic diseases, (ii) increasing the level of available chaperones may augment the level of functional mutant protein in vivo, and (iii) one mutation may have multiple effects. The interplay between the chaperones assisting folding and proteases that attack folding intermediates is decisive for how large a proportion of a mutant polypeptide impaired in folding acquires the functional structure. This constitutes a protein quality control system, and the handling of a given mutant protein by this system may vary due to environmental conditions or genetic variability in its components. The possibility that intraindividual differences in the handling of mutant proteins may be a mechanism accounting for phenotypic variability is discussed.

A model for Batten disease protein CLN3: functional implications from homology and mutations

In an attempt to understand the molecular nature of Batten disease, we have examined the amino acid sequence of the affected CLN3 gene product (The International Batten Disease Consortium (1995) Cell 82, 949-957) and the site-specific mutations which give rise to the biological defect. Homology searches and molecular modeling have led to the development of a model for the folding and disposition of the protein, possibly within a mitochondrial membrane. High homology with a yeast protein of unknown function suggests a strong evolutionary conservation of function. We speculate that a possible role for the protein may be in chaperoning the folding/unfolding or assembly/ disassembly of other proteins, specifically subunit c of the mitochondrial ATP synthase complex.