genes associated with
calcium-independent phospholipase A2 VIA
of the disease
The PLA2G6 gene encodes an 85-kDa group VI calcium-independent phospholipase A2 beta (PLA2G6). This enzyme hydrolyses the sn-2 acyl chain of glycerophospholipids to release free fatty acids and lysophospholipids. PLA2G6 localizes to the mitochondria, and has proposed roles in the remodelling of membrane phospholipids, signal transduction and calcium signalling, cell proliferation and apoptosis. Humans with PLA2G6 mutations can show progressive cognitive and motor skill regression, as displayed in disorders such as infantile neuroaxonal dystrophy, neurodegeneration with brain iron accumulation type II and Karak syndrome. Infantile neuroaxonal dystrophy is a neurodegenerative disease with onset in infancy and fatality in the teenage years or in early adulthood. It is characterized neuropathologically by axonal swelling and the presence of spheroid bodies in the central and peripheral nervous systems in addition to hallmark cerebellar atrophy. Neurodegeneration with brain iron accumulation comprises a clinically and genetically heterogeneous group of disorders with a progressive extrapyramidal syndrome and high basal ganglia iron, and includes pantothenate kinase-associated neurodegeneration caused by mutations in PANK2 (neurodegeneration with brain iron accumulation type I). Post-mortem examination of the brain of a patient with neurodegeneration with brain iron accumulation associated with homozygous PLA2G6 mutations demonstrated both Parkinson’s and Alzheimer’s disease pathology with widespread Lewy bodies, dystrophic neurites and cortical neuronal neurofibrillary tangles (Kinghorn, 2015 and references therein, see below).
Furthermore, PLA2G6 has also been implicated in a number of other brain diseases including Alzheimer’s disease and bipolar disorder. More recently, PLA2G6, at the PARK14 locus, has also been characterized as the causative gene in a subgroup of patients with autosomal recessive early-onset dystonia-parkinsonism. Interestingly these patients do not display cerebellar atrophy or basal ganglia iron on MRI, and neuropathological examination reveals widespread Lewy body pathology and the accumulation of hyperphosphorylated tau. These clinical and neuropathological features further support a link between PLA2G6 mutations and parkinsonian disorders, and demonstrate the clinical heterogeneity in PLA2G6-associated neurodegeneration (Kinghorn, 2015 and references therein, see below).
It is not known how mutations in PLA2G6 cause neuropathology. However, it has been suggested that the loss of normal PLA2G6 activity in neurodegenerative disease involves structural abnormalities of cell or mitochondrial membranes and disturbances in lipid homeostasis and lipid metabolism. Indeed, a study in a mouse Pla2g6 knockout model demonstrates ultrastructural abnormalities in mitochondrial and synaptic membranes. As well as possible mitochondrial defects, it is thought that changes in the composition of the plasma membrane and other intracellular membranes may in turn affect the normal processes responsible for the movement of membranes within axons and dendrites, subsequently leading to accumulation of membranes as so-called spheroids (Kinghorn, 2015 and references therein, see below).
Support for a loss-of-function of PLA2G6 enzyme activity in causing disease comes from a recent study on recombinant wild-type and mutant human PLA2G6 proteins. This demonstrated that mutations in PLA2G6 associated with infantile neuroaxonal dystrophy or neurodegeneration with brain iron accumulation result in encoded proteins that exhibit <20% of control levels of phospholipase and lysophospholipase activities. Conversely, mutations associated with dystonia-parkinsonism do not impair catalytic activity. The differential enzymatic activities associated with PLA2G6 mutations in infantile neuroaxonal dystrophy/neurodegeneration with brain iron accumulation and dystonia-parkinsonism may explain the relatively later onset and milder phenotype seen in the latter. Furthermore, the preserved enzymatic function of dystonia-parkinsonism causing mutations must be interpreted with caution as there may be differences in PLA2G6 activity in vivo that are not detected by these in vitro assays. For example, there may be alternative mechanisms that alter the regulation of PLA2G6 activity, such as changes in the binding to calmodulin or other proteins. Further support for the loss of enzymatic function hypothesis comes from a recent study of a Chinese population with Parkinson’s disease, which identifies novel PLA2G6 mutations occurring in the heterozygous state, associated with an inhibition in the phospholipid-hydrolysing functions of PLA2G6. Moreover another study found a genotype–phenotype correlation in patients with infantile neuroaxonal dystrophy and neurodegeneration with brain iron accumulation: mutations that are predicted to lead to an absence of protein are associated with more severe infantile neuroaxonal dystrophy-type clinical phenotypes, while those with compound heterozygous missense mutations correlate with the less severe phenotype of neurodegeneration with brain iron accumulation and are predicted to result in protein with some residual enzyme function (Kinghorn, 2015 and references therein, see below).
Genetic ablation of Pla2g6 in mice is associated with an in vivo disturbance of brain phospholipid metabolism, mitochondrial membrane degeneration and marked cerebellar atrophy. Furthermore, in vitro experiments demonstrate a disturbance in calcium signalling in astrocytes from PLA2G6-deficient mice. However, despite these studies, the precise molecular mechanisms linking loss of PLA2G6 activity to mitochondrial morphological abnormalities and neurodegeneration remain unclear and further studies are required (Kinghorn, 2015 and references therein, see below).
Relevant studies of PLA2G6-associated neurodegeneration
Kinghorn, K.J., Castillo-Quan, J.I., Bartolome, F., Angelova, P.R., Li, L., Pope, S., Cochemé, H.M., Khan, S., Asghari, S., Bhatia, K.P., Hardy, J., Abramov, A.Y. and Partridge, L. (2015). Loss of PLA2G6 leads to elevated mitochondrial lipid peroxidation and mitochondrial dysfunction. Brain 138: 1801-1816. PubMed ID: 26001724
This study demonstrates that loss of normal phospholipase A2 activity in the fruit fly and in fibroblasts taken from patients harbouring mutations in PLA2G6, results in striking abnormalities in both mitochondrial function and morphology. Furthermore, it was demonstrated that mitochondrial dysfunction precedes the mitochondrial morphological abnormalities that are seen at the ultrastructural level. Deficits in mitochondrial oxidative respiration are seen in iPLA2-VIA knockout flies as young as 2 days of age, when there are no associated mitochondrial membrane abnormalities present on electron microscopy. Moreover, the finding that mitochondrial dysfunction occurs early in PLA2G6-associated neurodegeneration is consistent with the clinical features seen in infantile neuroaxonal dystrophy- neurodegeneration with brain iron accumulation and dystonia-parkinsonism, all of which display clear early-onset phenotypes, that have previously been associated with mitochondrial disease (Kinghorn, 2015).
In addition to mitochondrial dysfunction, it was also demonstrated that loss of PLA2G6 activity is associated with raised levels of reactive oxygen species. These species play important roles in cell signalling, but at abnormally high levels are detrimental to cells, ultimately leading to cell death. Lipids are important targets of reactive oxygen species and therefore elevated reactive oxygen species levels will lead to increases in mitochondrial lipid peroxidation. Furthermore, it may also be that elevated lipid peroxidation arises in the absence of phospholipase A2 activity because some phospholipases preferentially cleave oxidized lipids to maintain redox homeostasis. In turn, lipid peroxides are known to trigger a cascade of events that include a fall in mitochondrial membrane potential, mitochondrial swelling and loss of mitochondrial matrix components. Furthermore, it has been shown that complex I, the largest complex of the respiratory chain, is susceptible to oxidative stress. Indeed, a decrease in oxidative respiration in flies lacking iPLA2-VIA was observed when substrates specific for complex I of the respiratory chain were used. Furthermore, Lewy body disease has been shown to be associated with reduced levels of complex I activity, further implicating mitochondrial dysfunction in neurodegeneration, and more specifically in PLA2G6-associated neurodegeneration, which has been linked to parkinsonism and Lewy body pathology. In addition, lipid peroxidation is strongly catalysed through the Fenton reaction by iron and iron complexes and it is iron that is often deposited in the basal ganglia of the brains of patients affected with PLA2G6 mutations. Thus, iron-mediated oxidative insults seem to have a crucial role in the function of PLA2G6 and in the progression of diseases associated with mutations in this gene (Kinghorn, 2015).
Given the increase in levels of lipid peroxidation in flies lacking iPLA2-VIA, this study looked for elevated levels of oxidized cardiolipin, one of the most common lipids in the inner mitochondrial membrane. It has previously been hypothesized that PLA2G6 is involved in the remodelling of cardiolipin's fatty acid side chains and in removing oxidized fatty acid side chains. Oxidation of mitochondrial phospholipids such as cardiolipin leads to cytochrome c release and apoptosis. To determine whether cardiolipin is oxidized in PLA2G6 loss-of-function, the cardiolipin composition of iPLA2-VIA−/− fly heads was analyzed using mass spectrometry. It was demonstrated that mitochondrial membrane disintegration and mitochondrial dysfunction occur in the absence of any significant differences in cardiolipin amount or side chain composition in fly brains lacking iPLA2-VIA. Furthermore, oxidized cardiolipin was not detected in significant amounts. This may be related to the types of fatty acids present in Drosophila cardiolipin. Palmitic acid is the major component of cardiolipin side chains in Drosophila and is more resistant to oxidation than linoleic acid, the main cardiolipin side chain in most mammalian tissues. However, it could also point towards redundancy in the cardiolipin remodelling enzymes. There are a number of enzymes involved in cardiolipin remodelling and it may be the case that fatty acid remodelling of the side chains, in particular removal of oxidized fatty acids, can be performed by other enzymes involved in cardiolipin remodelling such as tafazzin (Kinghorn, 2015).
It was found that loss of normal PLA2G6 function is strongly associated with elevated mitochondrial lipid peroxidation. Furthermore lipid peroxidation is known to induce mitochondrial injury, leading to a decrease in oxidative respiratory chain function, reduced mitochondrial membrane potential, reduced cellular ATP levels and raised reactive oxygen species levels, all features that were demonstrated in flies and human fibroblasts lacking normal PLA2G6 gene activity. This in turn would be predicted to lead to cell death and neurodegeneration. As lipid peroxides are harmful mediators of oxidative stress, strategies focused on quenching, recycling or protecting such products could be highly effective in the treatment of the disease (Kinghorn, 2015).
In addition to showing that lipid peroxidation is increased in both fly brains lacking iPLA2-VIA and human PLA2G6 mutant fibroblasts, it was demonstrated that D-PUFAs reduce the rates of lipid peroxidation in two lines of PLA2G6 mutant fibroblasts and restore mitochondrial membrane potential. This suggests that D-PUFAs may have potential therapeutic roles in PLA2G6-associated neurodegeneration and its associated mitochondrial dysfunction (Kinghorn, 2015).
Overall, these data promote Drosophila as a useful model for studying PLA2G6-associated neurodegeneration, and for manipulating pathways and therapeutic targets in order to reverse or prevent the deleterious effects of lipid peroxides. Loss of iPLA2-VIA gene function results in reduced lifespan and locomotor ability, increased sensitivity to oxidative insults as well as hypoxia, osmotic and starvation stressors. Thus flies lacking iPLA2-VIA display phenotypes that can be used as markers in genetic modifying screens or in the screening of potential therapeutic compounds. To test the validity of the fly model for drug screening and to confirm the benefits of D-PUFAs in vivo in an organism lacking iPLA2-VIA, the effects of D-PUFAs on the locomotor deficits of iPLA2-VIA−/− flies were examined. This demonstrated that treatment with D2-linoleic acid partially rescues the climbing abnormalities in aged iPLA2-VIA knockout flies. Therefore this approach of using compounds, which act to reduce the autoxidation of lipids, may represent a future therapeutic strategy for patients with PLA2G6-associated neurodegeneration and warrants further investigation (Kinghorn, 2015).
Ramanadham, S., Ali, T., Ashley, J. W., Bone, R. N., Hancock, W. D. and Lei, X. (2015). Calcium-Independent Phospholipases A2 (iPLA2s) and their Roles in Biological Processes and Diseases. J Lipid Res. PubMed ID: 26023050
Implication of iPLA2-VIA in Barth syndrome
Go to top
Date revised: 10 August 2015
Home page: The Interactive Fly © 2015 Thomas Brody, Ph.D.
The Interactive Fly resides on the web server of the Society for Developmental Biology