Specific Delay of Degradation of Mitochondrial ATP Synthase Subunit c in Late Infantile Neuronal Ceroid Lipofuscinosis (Batten Disease)

Abstract

Subunit c is normally present as an inner mitochondrial membrane component of the F0 section of the ATP synthase complex, but in the late infantile form of neuronal ceroid lipofuscinosis (NCL) it was also found in lysosomes in high concentrations. To explore the mechanism of storage of subunit c, the rates of degradation and synthesis of subunit c were measured in fibroblast cell types from controls and patients with the late infantile form of NCL. The radiolabel from subunit c decreased with time in control cells, whereas no apparent loss of radioactivity of subunit c was found in patients' cells. There were no significant differences between control cells and cells with disease in the degradation of cytochrome oxidase subunit IV, an inner membrane protein of mitochondria. A combination of pulse‐chase and subcellular fractionation analysis showed that a delay of intramitochondrial loss from prelabeled subunit c was seen in all diseased cells tested. Lysosomal appearance of labeled subunit c could be detected after chase for more than 1 week and its radioactivities were variable among diseased cell types. The biosynthetic rate of subunit c was almost the same in both control and patient cells. Northern blotting analyses showed that mRNAs for P1 and P2 genes had no significant difference in lengths and amounts between control and patient cells. Results suggest a specific failure in the degradation of subunit c after its normal inclusion in mitochondria and its consequent accumulation in lysosomes. This is the first direct evidence to show a delay of subunit c degradation in the cells from the late infantile form of NCL.

How Mitochondrial Damage Affects Cell Function

“…The accumulation of such partially degraded material correlates with cellular ageing in post-mitotic cells such as neurons [99], and is accelerated in certain degenerative neurological conditions such as Alzheimer's [77], Batten's [19] and Huntington's diseases [53]. The hydrophobicity of the mtDNA-encoded subunits is likely to make them a difficult substrate for degradation by proteases as they may accumulate in insoluble bodies, as is the case for subunit c of the F 0 F 1 -ATPase [30,79]. While the nature of the undigested material in residual bodies is uncertain, it is thought to include oxidatively damaged material that cannot be digested by enzymes [99].…”

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

“…This complex consists of 14 different polypeptides that are selectively degraded with different half-lives, but subunit c is the only one of these peptides that abnormally accumulates in the lysosomal storage vesicles of LINCL. Other hydrophobic proteins such as the ␤ subunit of ATP synthase or cytochrome oxidase subunit IV were all degraded normally in LINCL (Ezaki et al, 1995(Ezaki et al, , 1999. Subunit c has also been reported to accumulate in other forms of NCL as well as several animal models of Batten's disease but the amounts are much less and small amounts of subunit c are found in other pathological lysosomes such as those in mucopolysaccharidosis (Elleder et al, 1997).…”

“…The metabolic study of subunit c using [ 35 S] methionine pulse labeling revealed that whereas synthesis was normal, the degradation of subunit c was delayed in LINCL (Ezaki et al, 1995). The defective degradation of subunit c in LINCL does not seem to result from any structural alterations in the protein or any posttranslational modification, because mitochondrial proteins isolated from [ 35 S] methionine-labeled LINCL cells were readily degraded by normal lysosomal extracts (Ezaki et al, 1996).…”

How mitochondrial damage affects cell function

“…The accumulation of such partially degraded material correlates with cellular ageing in post-mitotic cells such as neurons [99], and is accelerated in certain degenerative neurological conditions such as Alzheimer's [77], Batten's [19] and Huntington's diseases [53]. The hydrophobicity of the mtDNA-encoded subunits is likely to make them a difficult substrate for degradation by proteases as they may accumulate in insoluble bodies, as is the case for subunit c of the FoF1-ATPase [30,79]. While the nature of the undigested material in residual bodies is uncertain, it is thought to include oxidatively damaged material that cannot be digested by enzymes [99].…”