Establishment of an improved mouse model for infantile neuroaxonal dystrophy that shows early disease onset and bears a point mutation in Pla2g6

A missense mutation in the Hspa8 gene encoding heat shock cognate protein 70 causes neuroaxonal dystrophy in rats

Neuroaxonal dystrophy (NAD) is a neurodegenerative disease characterized by spheroid (swollen axon) formation in the nervous system. In the present study, we focused on a newly established autosomal recessive mutant strain of F344-kk/kk rats with hind limb gait abnormalities and ataxia from a young age. Histopathologically, a number of axonal spheroids were observed throughout the central nervous system, including the spinal cord (mainly in the dorsal cord), brain stem, and cerebellum in F344-kk/kk rats. Transmission electron microscopic observation of the spinal cord revealed accumulation of electron-dense bodies, degenerated abnormal mitochondria, as well as membranous or tubular structures in the axonal spheroids. Based on these neuropathological findings, F344-kk/kk rats were diagnosed with NAD. By a positional cloning approach, we identified a missense mutation (V95E) in the Hspa8 (heat shock protein family A (Hsp70) member 8) gene located on chromosome 8 of the F344-kk/kk rat genome. Furthermore, we developed the Hspa8 knock-in (KI) rats with the V95E mutation using the CRISPR-Cas system. Homozygous Hspa8-KI rats exhibited ataxia and axonal spheroids similar to those of F344-kk/kk rats. The V95E mutant HSC70 protein exhibited the significant but modest decrease in the maximum hydrolysis rate of ATPase when stimulated by co-chaperons DnaJB4 and BAG1 in vitro, which suggests the functional deficit in the V95E HSC70. Together, our findings provide the first evidence that the genetic alteration of the Hspa8 gene caused NAD in mammals.

Molecular basis of unique specificity and regulation of group VIA calcium-independent phospholipase A2 (PNPLA9) and its role in neurodegenerative diseases

Glycerophospholipids are major components of cell membranes and consist of a glycerol backbone esterified with one of over 30 unique fatty acids at each of the sn-1 and sn-2 positions. In addition, in some human cells and tissues as much as 20% of the glycerophospholipids contain a fatty alcohol rather than an ester in the sn-1 position, although it can also occur in the sn-2 position. The sn-3 position of the glycerol backbone contains a phosphodiester bond linked to one of more than 10 unique polar head-groups. Hence, humans contain thousands of unique individual molecular species of phospholipids given the heterogeneity of the sn-1 and sn-2 linkage and carbon chains and the sn-3 polar groups. Phospholipase A2 (PLA2) is a superfamily of enzymes that hydrolyze the sn-2 fatty acyl chain resulting in lyso-phospholipids and free fatty acids that then undergo further metabolism. PLA2's play a critical role in lipid-mediated biological responses and membrane phospholipid remodeling. Among the PLA2 enzymes, the Group VIA calcium-independent PLA2 (GVIA iPLA2), also referred to as PNPLA9, is a fascinating enzyme with broad substrate specificity and it is implicated in a wide variety of diseases. Especially notable, the GVIA iPLA2 is implicated in the sequelae of several neurodegenerative diseases termed "phospholipase A2-associated neurodegeneration" (PLAN) diseases. Despite many reports on the physiological role of the GVIA iPLA2, the molecular basis of its enzymatic specificity was unclear. Recently, we employed state-of-the-art lipidomics and molecular dynamics techniques to elucidate the detailed molecular basis of its substrate specificity and regulation. In this review, we summarize the molecular basis of the enzymatic action of GVIA iPLA2 and provide a perspective on future therapeutic strategies for PLAN diseases targeting GVIA iPLA2.

The phospholipase A2 superfamily as a central hub of bioactive lipids and beyond

In essence, "phospholipase A₂" (PLA₂) means a group of enzymes that release fatty acids and lysophospholipids by hydrolyzing the sn-2 position of glycerophospholipids. To date, more than 50 enzymes possessing PLA₂ or related lipid-metabolizing activities have been identified in mammals, and these are subdivided into several families in terms of their structures, catalytic mechanisms, tissue/cellular localizations, and evolutionary relationships. From a general viewpoint, the PLA₂ superfamily has mainly been implicated in signal transduction, driving the production of a wide variety of bioactive lipid mediators. However, a growing body of evidence indicates that PLA₂s also contribute to phospholipid remodeling or recycling for membrane homeostasis, fatty acid β-oxidation for energy production, and barrier lipid formation on the body surface. Accordingly, PLA₂ enzymes are considered one of the key regulators of a broad range of lipid metabolism, and perturbation of specific PLA₂-driven lipid pathways often disrupts tissue and cellular homeostasis and may be associated with a variety of diseases. This review covers current understanding of the physiological functions of the PLA₂ superfamily, focusing particularly on the two major intracellular PLA₂ families (Ca²⁺-dependent cytosolic PLA₂s and Ca²⁺-independent patatin-like PLA₂s) as well as other PLA₂ families, based on studies using gene-manipulated mice and human diseases in combination with comprehensive lipidomics.

Exploring therapeutic strategies for infantile neuronal axonal dystrophy (INAD/PARK14)

Infantile neuroaxonal dystrophy (INAD) is caused by recessive variants in PLA2G6 and is a lethal pediatric neurodegenerative disorder. Loss of the Drosophila homolog of PLA2G6, leads to ceramide accumulation, lysosome expansion, and mitochondrial defects. Here, we report that retromer function, ceramide metabolism, the endolysosomal pathway, and mitochondrial morphology are affected in INAD patient-derived neurons. We show that in INAD mouse models, the same features are affected in Purkinje cells, arguing that the neuropathological mechanisms are evolutionary conserved and that these features can be used as biomarkers. We tested 20 drugs that target these pathways and found that Ambroxol, Desipramine, Azoramide, and Genistein alleviate neurodegenerative phenotypes in INAD flies and INAD patient-derived neural progenitor cells. We also develop an AAV-based gene therapy approach that delays neurodegeneration and prolongs lifespan in an INAD mouse model.

A GLP1 receptor agonist diabetes drug ameliorates neurodegeneration in a mouse model of infantile neurometabolic disease

Infantile neuroaxonal dystrophy (INAD) is a rare paediatric neurodegenerative condition caused by mutations in the PLA2G6 gene, which is also the causative gene for PARK14-linked young adult-onset dystonia parkinsonism. INAD patients usually die within their first decade of life, and there are currently no effective treatments available. GLP1 receptor (GLP-1R) agonists are licensed for treating type 2 diabetes mellitus but have also demonstrated neuroprotective properties in a clinical trial for Parkinson's disease. Therefore, we evaluated the therapeutic efficacy of a new recently licensed GLP-1R agonist diabetes drug in a mouse model of INAD. Systemically administered high-dose semaglutide delivered weekly to juvenile INAD mice improved locomotor function and extended the lifespan. An investigation into the mechanisms underlying these therapeutic effects revealed that semaglutide significantly increased levels of key neuroprotective molecules while decreasing those involved in pro-neurodegenerative pathways. The expression of mediators in both the apoptotic and necroptotic pathways were also significantly reduced in semaglutide treated mice. A reduction of neuronal loss and neuroinflammation was observed. Finally, there was no obvious inflammatory response in wild-type mice associated with the repeated high doses of semaglutide used in this study.

Novel PLA2G6 Pathogenic Variants in Chinese Patients With PLA2G6-Associated Neurodegeneration

Background

PLA2G6-associated neurodegeneration (PLAN) is a heterogeneous group of neurodegenerative diseases caused by biallelic PLA2G6 mutations, covering diseases such as infantile neuroaxonal dystrophy (INAD), atypical neuroaxonal dystrophy (ANAD), dystonia parkinsonism (DP), and autosomal recessive early-onset parkinsonism (AREP). The study aims to report the clinical and genetic features of a series of PLAN patients.

Methods

The clinical and radiological findings of five Chinese patients from three families were collected. Whole-exome next generation sequencing (NGS) was applied to identify the genetic causes. Co-segregation analysis of the detected candidate variants were performed in their families. The pathogenicity of identified novel variants was predicted by in silico analysis.

Results

NGS revealed compound heterozygous variants of PLA2G6 gene in all five patients. There were six PLA2G6 variants identified, including two known variants (c.116G>A, c.238G>A) and four novel variants (c.2120dupA, c.2071C>G, c.967G>A, c1534T>A). ACMG predicts c.2120dupA to be pathogenic, c.2071C>G and c.1534T>A to be likely pathogenic, and c1534T>A to be of uncertain significance. Clinically, four patients fell into the diagnosis of ANAD, and 1 into the diagnosis of AREP. Brain imaging revealed cerebellar atrophy, iron deposition in bilateral globus pallidus, and substantia nigra in three cases.

Conclusions

Four novel pathogenic variants were discovered and the pathogenic variant spectrum of the PLA2G6 gene was expanded.

iPLA2-VIA is required for healthy aging of neurons, muscle, and the female germline in Drosophila melanogaster

Neurodegenerative disease (ND) is a growing health burden worldwide, but its causes and treatments remain elusive. Although most cases of ND are sporadic, rare familial cases have been attributed to single genes, which can be investigated in animal models. We have generated a new mutation in the calcium-independent phospholipase A₂ (iPLA₂) VIA gene CG6718, the Drosophila melanogaster ortholog of human PLA2G6/PARK14, mutations in which cause a suite of NDs collectively called PLA2G6-associated neurodegeneration (PLAN). Our mutants display age-related loss of climbing ability, a symptom of neurodegeneration in flies. Although phospholipase activity commonly is presumed to underlie iPLA₂-VIA function, locomotor decline in our mutants is rescued by a transgene carrying a serine-to-alanine mutation in the catalytic residue, suggesting that important functional aspects are independent of phospholipase activity. Additionally, we find that iPLA₂-VIA knockdown in either muscle or neurons phenocopies locomotor decline with age, demonstrating its necessity in both neuronal and non-neuronal tissues. Furthermore, RNA in situ hybridization shows high endogenous iPLA₂-VIA mRNA expression in adult germ cells, and transgenic HA-tagged iPLA₂-VIA colocalizes with mitochondria there. Mutant males are fertile with normal spermatogenesis, while fertility is reduced in mutant females. Mutant female germ cells display age-related mitochondrial aggregation, loss of mitochondrial potential, and elevated cell death. These results suggest that iPLA₂-VIA is critical for mitochondrial integrity in the Drosophila female germline, which may provide a novel context to investigate its functions with parallels to PLAN.

Loss of tyrosine hydroxylase, motor deficits and elevated iron in a mouse model of phospholipase A2G6-associated neurodegeneration (PLAN)

Phospholipase A2G6-associated neurodegeneration (PLAN) is a rare early-onset monogenic neurodegenerative movement disorder which targets the basal ganglia and other regions in the central and peripheral nervous system; presenting as a series of heterogenous subtypes in patients. We describe here a B6.C3-Pla2g6^m1J/CxRwb mouse model of PLAN which presents with early-onset neurodegeneration at 90 days which is analogous of the disease progression that is observed in PLAN patients. Homozygous mice had a progressively worsening motor deficit, which presented as tremors starting at 65 days and progressed to severe motor dysfunction and increased falls on the wire hang test at 90 days. This motor deficit positively correlated with a reduction in tyrosine hydroxylase (TH) protein expression in dopaminergic neurons of the substantia nigra (SN) without any neuronal loss. Fluorescence imaging of Thy1-YFP revealed spheroid formation in the SN. The spheroids in homozygous mice strongly mirrors those observed in patients and were demonstrated to correlate strongly with the motor deficits as measured by the wire hang test. The appearance of spheroids preceded TH loss and increased spheroid numbers negatively correlated with TH expression. Perls/DAB staining revealed the presence of iron accumulation within the SN of mice. This mouse model captures many of the major hallmarks of PLAN including severe-early onset neurodegeneration, a motor deficit that correlates directly to TH levels, spheroid formation and iron accumulation within the basal ganglia. Thus, this mouse line is a useful tool for further research efforts to improve understanding of how these disease mechanisms give rise to the disease presentations seen in PLAN patients as well as to test novel therapies.

iPLA2β and its role in male fertility, neurological disorders, metabolic disorders, and inflammation

The Ca²⁺-independent phospholipases, designated as group VI iPLA₂s, also referred to as PNPLAs due to their shared homology with patatin, include the β, γ, δ, ε, ζ, and η forms of the enzyme. The iPLA₂s are ubiquitously expressed, share a consensus GXSXG catalytic motif, and exhibit organelle/cell-specific localization. Among the iPLA₂s, iPLA₂β has received wide attention as it is recognized to be involved in membrane remodeling, cell proliferation, cell death, and signal transduction. Ongoing studies implicate participation of iPLA₂β in a variety of disease processes including cancer, cardiovascular abnormalities, glaucoma, and peridonditis. This review will focus on iPLA₂β and its links to male fertility, neurological disorders, metabolic disorders, and inflammation.

PLA2G6-associated neurodegeneration: Lessons from neurophysiological findings

BACKGROUND AND AIMS

Phospholipase A2 associated neurodegeneration (PLAN) is a heterogeneous autosomal recessive disorder caused by mutations in the ubiquitously expressed PLA2G6 gene. It is responsible for delayed brain iron accumulation and induces progressive psychomotor regression. We report the concomitant clinical, radiological and neurophysiological findings in PLAN patients in an attempt to determine the contribution of each test to guide diagnosis.

METHODS

Concomitant clinical, radiological, electroencephalographic (EEG) and electrodiagnostic testing (EDX) findings in a series of 8 consecutive genetically confirmed PLAN patients were collected.

RESULTS

All patients presented marked motor axonal loss, with decreased or absent distal compound muscle action potentials, acute and chronic denervation at needle electromyography, in contrast with preservation of sensory conduction. EEG showed high-amplitude fast activity in all patients aged above 15 months. Two patients showing severe neonatal hypotonia displayed atypical hypsarhythmia and epileptic spasms. Iron deposition in globus pallidus was observed in only two patients aged above 6 years.

CONCLUSIONS

Peripheral involvement is an early feature in PLAN recognizable by EDX at an earlier stage than typical iron accumulation in the brain. Furthermore, the association of West syndrome and axonal motor neuropathy may represent positive clues in favor of PLAN. This results emphasize the interest of early and repeated EDX.

Mutations in the Drosophila homolog of human PLA2G6 give rise to age-dependent loss of psychomotor activity and neurodegeneration

Infantile neuroaxonal dystrophy (INAD) is a fatal neurodegenerative disorder that typically begins within the first few years of life and leads to progressive impairment of movement and cognition. Several years ago, it was shown that >80% of patients with INAD have mutations in the phospholipase gene, PLA2G6. Interestingly, mutations in PLA2G6 are also causative in two other related neurodegenerative diseases, atypical neuroaxonal dystrophy and Dystonia-parkinsonism. While all three disorders give rise to similar defects in movement and cognition, some defects are unique to a specific disorder. At present, the cellular mechanisms underlying PLA2G6-associated neuropathology are poorly understood and there is no cure or treatment that can delay disease progression. Here, we show that loss of iPLA2-VIA, the Drosophila homolog of PLA2G6, gives rise to age-dependent defects in climbing and spontaneous locomotion. Moreover, using a newly developed assay, we show that iPLA2-VIA mutants also display impairments in fine-tune motor movements, motor coordination and psychomotor learning, which are distinct features of PLA2G6-associated disease in humans. Finally, we show that iPLA2-VIA mutants exhibit increased sensitivity to oxidative stress, progressive neurodegeneration and a severely reduced lifespan. Altogether, these data demonstrate that Drosophila iPLA2-VIA mutants provide a useful model to study human PLA2G6-associated neurodegeneration.

Ultrastructural features of canine neuroaxonal dystrophy in a Papillon dog

Neuroaxonal dystrophy (NAD) is a neurodegenerative disease characterized by severe axonal swelling (spheroids) throughout the nervous system. In dogs, NAD has been reported in several breeds and a missense mutation in PLA2G6 gene has recently been identified in the Papillon dog NAD. Here we performed ultrastructural analysis to clarify the detailed ultrastructural features of the Papillon dog NAD. Dystrophic axons consisted of accumulation of filamentous materials, tubulovesicular structures, and swollen edematous mitochondria with degenerated inner membranes were often observed in the central nervous system. At axonal terminals, degeneration of presynaptic membrane was also detected. As reported in Pla2g6 knockout mice, mitochondrial and presynaptic degeneration may be related with the pathogenesis of NAD in Papillon dogs.

Identification of the PLA2G6 c.1579G>A Missense Mutation in Papillon Dog Neuroaxonal Dystrophy Using Whole Exome Sequencing Analysis

Whole exome sequencing (WES) has become a common tool for identifying genetic causes of human inherited disorders, and it has also recently been applied to canine genome research. We conducted WES analysis of neuroaxonal dystrophy (NAD), a neurodegenerative disease that sporadically occurs worldwide in Papillon dogs. The disease is considered an autosomal recessive monogenic disease, which is histopathologically characterized by severe axonal swelling, known as “spheroids,” throughout the nervous system. By sequencing all eleven DNA samples from one NAD-affected Papillon dog and her parents, two unrelated NAD-affected Papillon dogs, and six unaffected control Papillon dogs, we identified 10 candidate mutations. Among them, three candidates were determined to be “deleterious” by in silico pathogenesis evaluation. By subsequent massive screening by TaqMan genotyping analysis, only the PLA2G6 c.1579G>A mutation had an association with the presence or absence of the disease, suggesting that it may be a causal mutation of canine NAD. As a human homologue of this gene is a causative gene for infantile neuroaxonal dystrophy, this canine phenotype may serve as a good animal model for human disease. The results of this study also indicate that WES analysis is a powerful tool for exploring canine hereditary diseases, especially in rare monogenic hereditary diseases.

Validation of the finding of hypertrophy of the clava in infantile neuroaxonal dystrophy/PLA2G6 by biometric analysis

INTRODUCTION

Infantile neuroaxonal dystrophy (INAD), an autosomal recessive neurodegenerative disorder due to PLA2G6 mutation, is classified both as a PLA2G6-associated neurodegeneration (PLAN) disorder and as one of the neurodegeneration with brain iron accumulation (NBIA) disorders. Age of onset and clinical presentation in INAD is variable. Typically described imaging features of cerebellar atrophy, cerebellar cortex bright FLAIR signal, and globus pallidus iron deposition are variable or late findings. We characterize clinical and neuroimaging phenotypes in nine children with confirmed PLA2G6 mutations and show a useful imaging feature, clava hypertrophy, which may aid in earlier identification of patients. Measurements of the clava confirm actual enlargement, rather than apparent enlargement due to volume loss of the other brain stem structures.

METHODS

A retrospective clinical and MRI review was performed. Brain stem measurements were performed and compared with age-matched controls.

RESULTS

We identified nine patients, all with novel PLA2G6 gene mutations. MRI, available in eight, showed clava hypertrophy, regardless of age or the absence of other more typically described neuroimaging findings. Brain autopsy in our cohort confirmed prominent spheroid bodies in the clava nuclei.

CONCLUSION

Clava hypertrophy is an important early imaging feature which may aid in indentification of children who would benefit from specific testing for PLA2G6 mutations.

Identification of a novel mutation in PLA2G6 gene in a Chinese pedigree with familial cortical myoclonic tremor with epilepsy

PURPOSE

Familial cortical myoclonic tremor with epilepsy (FCMTE) is an epileptic syndrome with autosomal dominant inheritance, of which four genetic subtypes (FCMTE1-4) have been reported. In the present study, we described the clinical and neurophysiologic features of a newly diagnosed Chinese FCMTE family, and investigated the genetic cause for this disease.

METHODS

Clinical information was obtained from affected and normal individuals of an FCMTE family comprising 41 members. Electroencephalographies were analyzed in five of six affected members (including the proband). Brain magnetic resonance imaging, somatosensory evoked potential with C-reflex analysis and magnetoencephalography was performed in the proband. Genomic DNA of three affected and two unaffected individuals was analyzed to detect the genetic mutations by using whole-exome sequencing.

RESULTS

The inheritance pattern of the pedigree was autosomal dominant. A novel missense mutation c.475C>T (p.Ala159Thr) of PLA2G6 were identified in this family. The mutated locus is highly conserved among other species. The mutation is predicted to have a functional impact, and completely co-segregated with the phenotype.

CONCLUSION

This study identifies a novel PLA2G6 mutation that is the possible genetic cause of FCMTE in this family. This mutation and associated clinical features expand the spectrum and phenotypes of PLA2G6-related disorders including neurodegenerative diseases.

Identification of Novel Compound Mutations in PLA2G6-Associated Neurodegeneration Patient with Characteristic MRI Imaging

Neurodegeneration with brain iron accumulation comprises a heterogeneous group of disorders characterized clinically by progressive motor dysfunction. Accurate identification of de novo and rare inherited mutations is important for determining causative genes of undiagnosed neurological diseases. In the present study, we report a unique case with cerebellar ataxia symptoms and social communication difficulties in an intermarriage family. MRI showed a marked cerebellar atrophy and the "eye-of-the-tiger"-like sign in the medial globus pallidus. Potential genetic defects were screened by whole-exome sequencing (WES) for the patient and four additional family members. A previously undescribed de novo missense mutation (c.1634A>G, p.K545R) in the exon 12 of the PLA2G6 gene was identified. A second rare variant c.1077G>A at the end of exon 7 was also identified, which was inherited from the mother, and resulted in a frame-shift mutation (c.1074_1077del.GTCG) due to an alternative splicing. In conclusion, the identification of the "eye-of-the-tiger"-like sign in the globus pallidus of the patient expands the phenotypic spectrum of PLA2G6-associated disorders and reveals its value in differential diagnosis of PLA2G6-associated disorders.

Genetic Analysis of PLA2G6 in 22 Indian Families with Infantile Neuroaxonal Dystrophy, Atypical Late-Onset Neuroaxonal Dystrophy and Dystonia Parkinsonism Complex

Mutations in PLA2G6 were identified in patients with a spectrum of neurodegenerative conditions, such as infantile neuroaxonal dystrophy (INAD), atypical late-onset neuroaxonal dystrophy (ANAD) and dystonia parkinsonism complex (DPC). However, there is no report on the genetic analysis of families with members affected with INAD, ANAD and DPC from India. Therefore, the main aim of this study was to perform genetic analysis of 22 Indian families with INAD, ANAD and DPC. DNA sequence analysis of the entire coding region of PLA2G6 identified 13 different mutations, including five novel ones (p.Leu224Pro, p.Asp283Asn, p.Arg329Cys, p.Leu491Phe, and p.Arg649His), in 12/22 (54.55%) families with INAD and ANAD. Interestingly, one patient with INAD was homozygous for two different mutations, p.Leu491Phe and p.Ala516Val, and thus harboured four mutant alleles. With these mutations, the total number of mutations in this gene reaches 129. The absence of mutations in 10/22 (45.45%) families suggests that the mutations could be in deep intronic or promoter regions of this gene or these families could have mutations in a yet to be identified gene. The present study increases the mutation landscape of PLA2G6. The present finding will be useful for genetic diagnosis, carrier detection and genetic counselling to families included in this study and other families with similar disease condition.

Mitochondrial dysfunction and defects in lipid homeostasis as therapeutic targets in neurodegeneration with brain iron accumulation

The PLA2G6 gene encodes a group VIA calcium independent phospholipase A2 (iPLA2β), which hydrolyses glycerophospholipids to release fatty acids and lysophospholipids. Mutations in PLA2G6 are associated with a number of neurodegenerative disorders including neurodegeneration with brain iron accumulation (NBIA), infantile neuroaxonal dystrophy (INAD), and dystonia parkinsonism, collectively known as PLA2G6-associated neurodegeneration (PLAN). Recently Kinghorn et al. demonstrated in Drosophila and PLA2G6 mutant fibroblasts that loss of normal PLA2G6 activity is associated with mitochondrial dysfunction and mitochondrial lipid peroxidation. Furthermore, they were able to show the beneficial effects of deuterated polyunsaturated fatty acids (D-PUFAs), which reduce lipid peroxidation. D-PUFAs were able to rescue the locomotor deficits of flies lacking the fly ortholog of PLA2G6 (iPLA2-VIA), as well as the mitochondrial abnormalities in PLA2G6 mutant fibroblasts. This work demonstrated that the iPLA2-VIA knockout fly is a useful organism to dissect the mechanisms of pathogenesis of PLAN, and that further investigation is required to determine the therapeutic potential of D-PUFAs in patients with PLA2G6 mutations. The fruit fly has also been used to study some of the other genetic causes of NBIA, and here we also describe what is known about the mechanisms of pathogenesis of these NBIA variants. Mitochondrial dysfunction, defects in lipid metabolism, as well as defective Coenzyme A (CoA) biosynthesis, have all been implicated in some genetic forms of NBIA, including PANK2, CoASY, C12orf19 and FA2H.

Impairment of PARK14-dependent Ca(2+) signalling is a novel determinant of Parkinson's disease

The etiology of idiopathic Parkinson's disease (idPD) remains enigmatic despite recent successes in identification of genes (PARKs) that underlie familial PD. To find new keys to this incurable neurodegenerative disorder we focused on the poorly understood PARK14 disease locus (Pla2g6 gene) and the store-operated Ca(2+) signalling pathway. Analysis of the cells from idPD patients reveals a significant deficiency in store-operated PLA2g6-dependent Ca(2+) signalling, which we can mimic in a novel B6.Cg-Pla2g6(ΔEx2-VB) (PLA2g6 ex2(KO)) mouse model. Here we demonstrate that genetic or molecular impairment of PLA2g6-dependent Ca(2+) signalling is a trigger for autophagic dysfunction, progressive loss of dopaminergic (DA) neurons in substantia nigra pars compacta and age-dependent L-DOPA-sensitive motor dysfunction. Discovery of this previously unknown sequence of pathological events, its association with idPD and our ability to mimic this pathology in a novel genetic mouse model opens new opportunities for finding a cure for this devastating neurodegenerative disease.

TECPR2 Associated Neuroaxonal Dystrophy in Spanish Water Dogs

Clinical, pathological and genetic examination revealed an as yet uncharacterized juvenile-onset neuroaxonal dystrophy (NAD) in Spanish water dogs. Affected dogs presented with various neurological deficits including gait abnormalities and behavioral deficits. Histopathology demonstrated spheroid formation accentuated in the grey matter of the cerebral hemispheres, the cerebellum, the brain stem and in the sensory pathways of the spinal cord. Iron accumulation was absent. Ultrastructurally spheroids contained predominantly closely packed vesicles with a double-layered membrane, which were characterized as autophagosomes using immunohistochemistry. The family history of the four affected dogs suggested an autosomal recessive inheritance. SNP genotyping showed a single genomic region of extended homozygosity of 4.5 Mb in the four cases on CFA 8. Linkage analysis revealed a maximal parametric LOD score of 2.5 at this region. By whole genome re-sequencing of one affected dog, a perfectly associated, single, non-synonymous coding variant in the canine tectonin beta-propeller repeat-containing protein 2 (TECPR2) gene affecting a highly conserved region was detected (c.4009C>T or p.R1337W). This canine NAD form displays etiologic parallels to an inherited TECPR2 associated type of human hereditary spastic paraparesis (HSP). In contrast to the canine NAD, the spinal cord lesions in most types of human HSP involve the sensory and the motor pathways. Furthermore, the canine NAD form reveals similarities to cases of human NAD defined by widespread spheroid formation without iron accumulation in the basal ganglia. Thus TECPR2 should also be considered as candidate gene for human NAD. Immunohistochemistry and the ultrastructural findings further support the assumption, that TECPR2 regulates autophagosome accumulation in the autophagic pathways. Consequently, this report provides the first genetic characterization of juvenile canine NAD, describes the histopathological features associated with the TECPR2 mutation and provides evidence to emphasize the association between failure of autophagy and neurodegeneration.

Calcium-independent phospholipases A2 and their roles in biological processes and diseases

Among the family of phospholipases A2 (PLA2s) are the Ca(2+)-independent PLA2s (iPLA2s) and they are designated group VI iPLA2s. In relation to secretory and cytosolic PLA2s, the iPLA2s are more recently described and details of their expression and roles in biological functions are rapidly emerging. The iPLA2s or patatin-like phospholipases (PNPLAs) are intracellular enzymes that do not require Ca(2+) for activity, and contain lipase (GXSXG) and nucleotide-binding (GXGXXG) consensus sequences. Though nine PNPLAs have been recognized, PNPLA8 (membrane-associated iPLA2γ) and PNPLA9 (cytosol-associated iPLA2β) are the most widely studied and understood. The iPLA2s manifest a variety of activities in addition to phospholipase, are ubiquitously expressed, and participate in a multitude of biological processes, including fat catabolism, cell differentiation, maintenance of mitochondrial integrity, phospholipid remodeling, cell proliferation, signal transduction, and cell death. As might be expected, increased or decreased expression of iPLA2s can have profound effects on the metabolic state, CNS function, cardiovascular performance, and cell survival; therefore, dysregulation of iPLA2s can be a critical factor in the development of many diseases. This review is aimed at providing a general framework of the current understanding of the iPLA2s and discussion of the potential mechanisms of action of the iPLA2s and related involved lipid mediators.

Neurodegeneration with Brain Iron Accumulation: Genetic Diversity and Pathophysiological Mechanisms

Neurodegeneration with brain iron accumulation (NBIA) comprises a heterogeneous group of progressive disorders with the common feature of excessive iron deposition in the brain. Over the last decade, advances in sequencing technologies have greatly facilitated rapid gene discovery, and several single-gene disorders are now included in this group. Identification of the genetic bases of the NBIA disorders has advanced our understanding of the disease processes caused by reduced coenzyme A synthesis, impaired lipid metabolism, mitochondrial dysfunction, and defective autophagy. The contribution of iron to disease pathophysiology remains uncertain, as does the identity of a putative final common pathway by which the iron accumulates. Ongoing elucidation of the pathogenesis of each NBIA disorder will have significant implications for the identification and design of novel therapies to treat patients with these disorders.

Defective lipid metabolism in neurodegeneration with brain iron accumulation (NBIA) syndromes: not only a matter of iron

Neurodegeneration with brain iron accumulation (NBIA) is a group of devastating and life threatening rare diseases. Adult and early-onset NBIA syndromes are inherited as X-chromosomal, autosomal dominant or recessive traits and several genes have been identified as responsible for these disorders. Among the identified disease genes, only two code for proteins directly involved in iron metabolism while the remaining NBIA genes encode proteins with a wide variety of functions ranging from fatty acid metabolism and autophagy to still unknown activities. It is becoming increasingly evident that many neurodegenerative diseases are associated with metabolic dysfunction that often involves altered lipid metabolism. This is not surprising since neurons have a peculiar and heterogeneous lipid composition critical for the development and correct functioning of the nervous system. This review will focus on specific NBIA forms, namely PKAN, CoPAN, PLAN, FAHN and MPAN, which display an interesting link between neurodegeneration and alteration of phospholipids and sphingolipids metabolism, mitochondrial morphology and membrane remodelling.

PLA2G6-associated neurodegeneration (PLAN): further expansion of the clinical, radiological and mutation spectrum associated with infantile and atypical childhood-onset disease

Phospholipase A2 associated neurodegeneration (PLAN) is a major phenotype of autosomal recessive Neurodegeneration with Brain Iron Accumulation (NBIA). We describe the clinical phenotypes, neuroimaging features and PLA2G6 mutations in 5 children, of whom 4 presented with infantile neuroaxonal dystrophy (INAD). One other patient was diagnosed with the onset of PLAN in childhood, and our report highlights the diagnostic challenges associated with this atypical PLAN subtype. In this series, the neuroradiological relevance of classical PLAN features as well as apparent claval hypertrophy' is explored. Novel PLA2G6 mutations were identified in all patients. PLAN should be considered not only in patients presenting with a classic INAD phenotype but also in older patients presenting later in childhood with non-specific progressive neurological features including social communication difficulties, gait disturbance, dyspraxia, neuropsychiatric symptoms and extrapyramidal motor features.

Neurodegeneration with brain iron accumulation: update on pathogenic mechanisms

Perturbation of iron distribution is observed in many neurodegenerative disorders, including Alzheimer’s and Parkinson’s disease, but the comprehension of the metal role in the development and progression of such disorders is still very limited. The combination of more powerful brain imaging techniques and faster genomic DNA sequencing procedures has allowed the description of a set of genetic disorders characterized by a constant and often early accumulation of iron in specific brain regions and the identification of the associated genes; these disorders are now collectively included in the category of neurodegeneration with brain iron accumulation (NBIA). So far 10 different genetic forms have been described but this number is likely to increase in short time. Two forms are linked to mutations in genes directly involved in iron metabolism: neuroferritinopathy, associated to mutations in the FTL gene and aceruloplasminemia, where the ceruloplasmin gene product is defective. In the other forms the connection with iron metabolism is not evident at all and the genetic data let infer the involvement of other pathways: Pank2, Pla2G6, C19orf12, COASY, and FA2H genes seem to be related to lipid metabolism and to mitochondria functioning, WDR45 and ATP13A2 genes are implicated in lysosomal and autophagosome activity, while the C2orf37 gene encodes a nucleolar protein of unknown function. There is much hope in the scientific community that the study of the NBIA forms may provide important insight as to the link between brain iron metabolism and neurodegenerative mechanisms and eventually pave the way for new therapeutic avenues also for the more common neurodegenerative disorders. In this work, we will review the most recent findings in the molecular mechanisms underlining the most common forms of NBIA and analyze their possible link with brain iron metabolism.

Pathogenesis of autosomal dominant hereditary spastic paraplegia (SPG6) revealed by a rat model

Supplemental Digital Content is available in the text.

Hereditary spastic paraplegias (HSPs) are characterized by progressive spasticity and weakness in the lower extremities that result from length-dependent central to peripheral axonal degeneration. Mutations in the non-imprinted Prader-Willi/Angelman syndrome locus 1 (NIPA1) transmembrane protein cause an autosomal dominant form of HSP (SPG6). Here, we report that transgenic (Tg) rats expressing a human NIPA1/SPG6 mutation in neurons (Thy1.2-hNIPA1^G106R) show marked early onset behavioral and electrophysiologic abnormalities. Detailed morphologic analyses reveal unique histopathologic findings, including the accumulation of tubulovesicular organelles with endosomal features that start at axonal and dendritic terminals, followed by multifocal vacuolar degeneration in both the CNS and peripheral nerves. In addition, the NIPA1^G106R mutation in the spinal cord from older Tg rats results in an increase in bone morphogenetic protein type II receptor expression, suggesting that its degradation is impaired. This Thy1.2-hNIPA1^G106R Tg rat model may serve as a valuable tool for understanding endosomal trafficking in the pathogenesis of a subgroup of HSP with an abnormal interaction with bone morphogenetic protein type II receptor, as well as for developing potential therapeutic strategies for diseases with axonal degeneration and similar pathogenetic mechanisms.

Extensive aggregation of α-synuclein and tau in juvenile-onset neuroaxonal dystrophy: an autopsied individual with a novel mutation in the PLA2G6 gene-splicing site

BACKGROUND

Infantile neuroaxonal dystrophy (INAD) is a rare autosomal-recessive neurodegenerative disorder. Patients with INAD usually show neurological symptoms with infant onset and die in childhood. Recently, it was reported that mutations in the PLA2G6 gene cause INAD, but neuropathological analysis of genetically confirmed individuals with neuroaxonal dystrophy has been limited.

RESULTS

Here, we report a Japanese individual with neuroaxonal dystrophy associated with compound heterozygous mutations in the PLA2G6 gene. A novel splice-site mutation resulting in skipping and missense mutations (p.R538C) in exon 9 was identified in the patient. This patient initially presented with cerebellar ataxia at the age of 3 years, which was followed by symptoms of mental retardation, extrapyramidal signs, and epileptic seizure. The patient survived until 20 years of age. Neuropathological findings were characterized by numerous axonal spheroids, brain iron deposition, cerebellar neuronal loss, phosphorylated alpha-synuclein-positive Lewy bodies (LBs), and phosphorylated-tau-positive neurofibrillary tangles. In particular, LB pathology exhibited a unique distribution with extremely severe cortical involvement.

CONCLUSIONS

Our results support a genetic clinical view that compound heterozygous mutations with potential residual protein function are associated with a relatively mild phenotype. Moreover, the severe LB pathology suggests that dysfunction of the PLA2G6 gene primarily contributes to LB formation.

The neuropathology of neurodegeneration with brain iron accumulation

Neuropathology plays a key role in characterizing the pathogenesis of neurodegenerative diseases including forms of neurodegeneration with brain iron accumulation (NBIA). Despite important differences, several genetically diverse forms of NBIA nevertheless share common features in addition to iron deposition, such as the presence of neuroaxonal spheroids. Multiple forms of NBIA also demonstrate tau or synuclein pathology, suggesting parallels with both Alzheimer and Parkinson diseases. This chapter summarizes what has been learned from the study of human patient tissues. Gross and microscopic findings are delineated, and similarities and differences between forms of NBIA are presented. Neuropathologic findings often help characterize fundamental features of disease and provide a springboard for more focused hypothesis-driven studies. Lessons learned from neuropathology thus contribute much to the characterization of the molecular mechanisms of disease.

Four novel rare mutations of PLA2G6 in Chinese population with Parkinson's disease

BACKGROUND

Mutations in the phospholipase A2 Group 6 (PLA2G6) gene have been identified in autosomal recessive neurodegenerative diseases classified as infantile neuroaxonal dystrophy and neurodegeneration with brain iron accumulation. Recently, PLA2G6 was also reported as the causative gene for early-onset PARK14-linked dystonia-parkinsonism.

METHODS/RESULTS

To address whether PLA2G6 mutations are also an important cause of PD, we screened sequence variants of PLA2G6 in 250 PD patients and 550 controls in a Chinese Han populations. We identified four sequence changes: a coding synonymous c.1959T>A transition of exon13 in one patient, two missense mutations c.1966C>G in exon13 and c.2077C>G in exon14 in two different patients, which caused two amino acids change Leu656Val and Leu693Val respectively. We also found a frame-shift mutation P.His597fx69 in exon 12 in one patient. These four rare variants were not represented in 550 control individuals. Furthermore, we found that WT PLA2G6 enzyme hydrolyzed phospholipids while mutant PLA2G6 with P.His597fx69 frame-shift caused loss of enzyme activity, exhibiting less than 6% of the specific activity in phospholipase assays compared to that of WT PLA2G6. Mutant PLA2G6 with Leu656Val and Leu693Val decreased their activity by 45% and 35% in phospholipase assay respectively.

CONCLUSIONS

We identified four rare PLA2G6 mutations in 250 PD patients, enlarging the spectrum of PLA2G6 mutations in PD. Although PLA2G6 mutations account for only a small fraction of PD patients in Chinese populations, these mutations impair catalytic activity of their phospholipids-hydrolyzing function. These results indicate that PLA2G6 mutations maybe PD-causing in Chinese Han populations.

Neuronal phospholipid deacylation is essential for axonal and synaptic integrity

Recessively-inherited deficiency in the catalytic activity of calcium-independent phospholipase A2-beta (iPLA2β) and neuropathy target esterase (NTE) causes infantile neuroaxonal dystrophy and hereditary spastic paraplegia, respectively. Thus, these two related phospholipases have non-redundant functions that are essential for structural integrity of synapses and axons. Both enzymes are expressed in essentially all neurons and also have independent roles in glia. iPLA2β liberates sn-2 fatty acid and lysophospholipids from diacyl-phospholipids. Ca(2+)-calmodulin tonically-inhibits iPLA2β, but this can be alleviated by oleoyl-CoA. Together with fatty acyl-CoA-mediated conversion of lysophospholipid to diacyl-phospholipid this may regulate sn-2 fatty acyl composition of phospholipids. In the nervous system, iPLA2β is especially important for the turnover of polyunsaturated fatty acid-associated phospholipid at synapses. More information is required on the interplay between iPLA2β and iPLA2-gamma in deacylation of neuronal mitochondrial phospholipids. NTE reduces levels of phosphatidylcholine (PtdCho) by degrading it to glycerophosphocholine and two free fatty acids. The substrate for NTE may be nascent PtdCho complexed with a phospholipid-binding protein. Protein kinase A-mediated phosphorylation enhances PtdCho synthesis and may allow PtdCho accumulation by coordinate inhibition of NTE activity. NTE operates primarily at the endoplasmic reticulum in neuronal soma but is also present in axons. NTE-mediated PtdCho homeostasis facilitates membrane trafficking and this appears most critical for the integrity of axon terminals in the spinal cord and hippocampus. For maintenance of peripheral nerve axons, iPLA2β activity may be able to compensate for NTE-deficiency but not vice-versa. Whether agonists acting at neuronal receptors modulate the activity of either enzyme remains to be determined. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Brain, blood, and iron: perspectives on the roles of erythrocytes and iron in neurodegeneration

The terms "neuroacanthocytosis" (NA) and "neurodegeneration with brain iron accumulation" (NBIA) both refer to groups of genetically heterogeneous disorders, classified together due to similarities of their phenotypic or pathological findings. Even collectively, the disorders that comprise these sets are exceedingly rare and challenging to study. The NBIA disorders are defined by their appearance on brain magnetic resonance imaging, with iron deposition in the basal ganglia. Clinical features vary, but most include a movement disorder. New causative genes are being rapidly identified; however, the mechanisms by which mutations cause iron accumulation and neurodegeneration are not well understood. NA syndromes are also characterized by a progressive movement disorder, accompanied by cognitive and psychiatric features, resulting from mutations in a number of genes whose roles are also basically unknown. An overlapping feature of the two groups, NBIA and NA, is the occurrence of acanthocytes, spiky red cells with a poorly-understood membrane dysfunction. In this review we summarise recent developments in this field, specifically insights into cellular mechanisms and from animal models. Cell membrane research may shed light upon the significance of the erythrocyte abnormality, and upon possible connections between the two sets of disorders. Shared pathophysiologic mechanisms may lead to progress in the understanding of other types of neurodegeneration.

Severe disturbance in the Ca2+ signaling in astrocytes from mouse models of human infantile neuroaxonal dystrophy with mutated Pla2g6

Infantile neuroaxonal dystrophy (INAD; OMIM #no. 256600) is an inherited degenerative nervous system disorder characterized by nerve abnormalities in brain, spinal cord and peripheral nerves. About 85% of INAD patients carry mutations in the PLA2G6 gene that encodes for a Ca(2+)-independent phospholipase A(2) (VIA iPLA(2)), but how these mutations lead to disease is unknown. Besides regulating phospholipid homeostasis, VIA iPLA(2) is emerging with additional non-canonical functions, such as modulating store-regulated Ca(2+) entry into cells, and mitochondrial functions. In turn, defective Ca(2+) regulation could contribute to the development of INAD. Here, we studied possible changes in ATP-induced Ca(2+) signaling in astrocytes derived from two mutant strains of mice. The first strain carries a hypomorphic allele of the Pla2g6 that reduces transcript levels to 5-10% of that observed in wild-type mice. The second strain carries a point mutation in Pla2g6 that results in inactive VIA iPLA(2) protein with postulated gain in toxicity. Homozygous mice from both strains develop pathology analogous to that observed in INAD patients. The nucleotide ATP is the most important transmitter inducing Ca(2+) signals in astroglial networks. We demonstrate here a severe disturbance in Ca(2+) responses to ATP in astrocytes derived from both mutant mouse strains. The duration of the Ca(2+) responses in mutant astrocytes was significantly reduced when compared with values observed in control cells. We also show that the reduced Ca(2+) responses are probably due to a reduction in capacitative Ca(2+) entry (2.3-fold). Results suggest that altered Ca(2+) signaling could be a central mechanism in the development of INAD pathology.

Neuroaxonal dystrophy in calcium-independent phospholipase A2β deficiency results from insufficient remodeling and degeneration of mitochondrial and presynaptic membranes

Infantile neuroaxonal dystrophy (INAD) is a fatal neurodegenerative disease characterized by the widespread presence of axonal swellings (spheroids) in the CNS and PNS and is caused by gene abnormality in PLA2G6 [calcium-independent phospholipase A(2)β (iPLA(2)β)], which is essential for remodeling of membrane phospholipids. To clarify the pathomechanism of INAD, we pathologically analyzed the spinal cords and sciatic nerves of iPLA(2)β knock-out (KO) mice, a model of INAD. At 15 weeks (preclinical stage), periodic acid-Schiff (PAS)-positive granules were frequently observed in proximal axons and the perinuclear space of large neurons, and these were strongly positive for a marker of the mitochondrial outer membrane and negative for a marker of the inner membrane. By 100 weeks (late clinical stage), PAS-positive granules and spheroids had increased significantly in the distal parts of axons, and ultrastructural examination revealed that these granules were, in fact, mitochondria with degenerative inner membranes. Collapse of mitochondria in axons was accompanied by focal disappearance of the cytoskeleton. Partial membrane loss at axon terminals was also evident, accompanied by degenerative membranes in the same areas. Imaging mass spectrometry showed a prominent increase of docosahexaenoic acid-containing phosphatidylcholine in the gray matter, suggesting insufficient membrane remodeling in the presence of iPLA(2)β deficiency. Prominent axonal degeneration in neuroaxonal dystrophy might be explained by the collapse of abnormal mitochondria after axonal transportation. Insufficient remodeling and degeneration of mitochondrial inner membranes and presynaptic membranes appear to be the cause of the neuroaxonal dystrophy in iPLA(2)β-KO mice.

A novel mitofusin 2 mutation causes canine fetal-onset neuroaxonal dystrophy

We recently reported autosomal recessive fetal-onset neuroaxonal dystrophy (FNAD) in a large family of dogs that is not caused by mutation in the PLA2G6 locus (Fyfe et al., J Comp Neurol 518:3771-3784, 2010). Here, we report a genome-wide linkage analysis using 333 microsatellite markers to map canine FNAD to the telomeric end of chromosome 2. The interval of zero recombination was refined by single-nucleotide polymorphism (SNP) haplotype analysis to ~200 kb, and the included genes were sequenced. We found a homozygous 3-nucleotide deletion in exon 14 of mitofusin 2 (MFN2), predicting loss of a glutamate residue at position 539 in the protein of affected dogs. RT-PCR demonstrated near normal expression of the mutant mRNA, but MFN2 expression was undetectable to very low on western blots of affected dog brainstem, cerebrum, kidney, and cultured fibroblasts and by immunohistochemistry on brainstem sections. MFN2 is a multifunctional, membrane-bound GTPase of mitochondria and endoplasmic reticulum most commonly associated with human Charcot-Marie-Tooth disease type 2A2. The canine disorder extends the range of MFN2-associated phenotypes and suggests MFN2 as a candidate gene for rare cases of human FNAD.

Childhood disorders of neurodegeneration with brain iron accumulation (NBIA)

Neurodegeneration with brain iron accumulation (NBIA) comprises a heterogeneous group of progressive complex motor disorders characterized by the presence of high brain iron, particularly within the basal ganglia. A number of autosomal recessive NBIA syndromes can present in childhood, most commonly pantothenate kinase-associated neurodegeneration (PKAN; due to mutations in the PANK2 gene) and phospholipase A2 group 6-associated neurodegeneration (PLAN; associated with genetic defects in PLA2G6). Mutations in the genes that cause these two neuroaxonal dystrophies are thought to disrupt the normal cellular functions of phospholipid remodelling and fatty acid metabolism. A significant proportion of children with an NBIA phenotype have no genetic diagnosis and there are, no doubt, additional as yet undiscovered genes that account for a number of these cases. NBIA disorders can be diagnostically challenging as there is often phenotypic overlap between the different disease entities. This review aims to define the clinical, radiological, and genetic features of such disorders, providing the clinician with a stepwise approach to appropriate neurological and genetic investigation, as well as a clinical management strategy for these neurodegenerative syndromes.

Random mutagenesis of the mouse genome: a strategy for discovering gene function and the molecular basis of disease

Mutagenesis of mice with N-ethyl-N-nitrosourea (ENU) is a phenotype-driven approach to unravel gene function and discover new biological pathways. Phenotype-driven approaches have the advantage of making no assumptions about the function of genes and their products and have been successfully applied to the discovery of novel gene-phenotype relationships in many physiological systems. ENU mutagenesis of mice is used in many large-scale and more focused projects to generate and identify novel mouse models for the study of gene functions and human disease. This review examines the strategies and tools used in ENU mutagenesis screens to efficiently generate and identify functional mutations.

Recent progress in phospholipase A₂ research: from cells to animals to humans

Mammalian genomes encode genes for more than 30 phospholipase A₂s (PLA₂s) or related enzymes, which are subdivided into several classes including low-molecular-weight secreted PLA₂s (sPLA₂s), Ca²+-dependent cytosolic PLA₂s (cPLA₂s), Ca²+-independent PLA₂s (iPLA₂s), platelet-activating factor acetylhydrolases (PAF-AHs), lysosomal PLA₂s, and a recently identified adipose-specific PLA. Of these, the intracellular cPLA₂ and iPLA₂ families and the extracellular sPLA₂ family are recognized as the "big three". From a general viewpoint, cPLA₂α (the prototypic cPLA₂ plays a major role in the initiation of arachidonic acid metabolism, the iPLA₂ family contributes to membrane homeostasis and energy metabolism, and the sPLA₂ family affects various biological events by modulating the extracellular phospholipid milieus. The cPLA₂ family evolved along with eicosanoid receptors when vertebrates first appeared, whereas the diverse branching of the iPLA₂ and sPLA₂ families during earlier eukaryote development suggests that they play fundamental roles in life-related processes. During the past decade, data concerning the unexplored roles of various PLA₂ enzymes in pathophysiology have emerged on the basis of studies using knockout and transgenic mice, the use of specific inhibitors, and information obtained from analysis of human diseases caused by mutations in PLA₂ genes. This review focuses on current understanding of the emerging biological functions of PLA₂s and related enzymes.

Catalytic function of PLA2G6 is impaired by mutations associated with infantile neuroaxonal dystrophy but not dystonia-parkinsonism

BACKGROUND

Mutations in the PLA2G6 gene have been identified in autosomal recessive neurodegenerative diseases classified as infantile neuroaxonal dystrophy (INAD), neurodegeneration with brain iron accumulation (NBIA), and dystonia-parkinsonism. These clinical syndromes display two significantly different disease phenotypes. NBIA and INAD are very similar, involving widespread neurodegeneration that begins within the first 1-2 years of life. In contrast, patients with dystonia-parkinsonism present with a parkinsonian movement disorder beginning at 15 to 30 years of age. The PLA2G6 gene encodes the PLA2G6 enzyme, also known as group VIA calcium-independent phospholipase A(2), which has previously been shown to hydrolyze the sn-2 acyl chain of phospholipids, generating free fatty acids and lysophospholipids.

METHODOLOGY/PRINCIPAL FINDINGS

We produced purified recombinant wildtype (WT) and mutant human PLA2G6 proteins and examined their catalytic function using in vitro assays with radiolabeled lipid substrates. We find that human PLA2G6 enzyme hydrolyzes both phospholipids and lysophospholipids, releasing free fatty acids. Mutations associated with different disease phenotypes have different effects on catalytic activity. Mutations associated with INAD/NBIA cause loss of enzyme activity, with mutant proteins exhibiting less than 20% of the specific activity of WT protein in both lysophospholipase and phospholipase assays. In contrast, mutations associated with dystonia-parkinsonism do not impair catalytic activity, and two mutations produce a significant increase in specific activity for phospholipid but not lysophospholipid substrates.

CONCLUSIONS/SIGNIFICANCE

These results indicate that different alterations in PLA2G6 function produce the different disease phenotypes of NBIA/INAD and dystonia-parkinsonism. INAD/NBIA is caused by loss of the ability of PLA2G6 to catalyze fatty acid release from phospholipids, which predicts accumulation of PLA2G6 phospholipid substrates and provides a mechanistic explanation for the accumulation of membranes in neuroaxonal spheroids previously observed in histopathological studies of INAD/NBIA. In contrast, dystonia-parkinsonism mutations do not appear to directly impair catalytic function, but may modify substrate preferences or regulatory mechanisms for PLA2G6.

Expression of PLA2G6 in human fetal development: Implications for infantile neuroaxonal dystrophy

Mutations in PLA2G6, which encodes calcium-independent phospholipase A(2) group VIA (iPLA2-VIA), underlie the autosomal recessive disorder infantile neuroaxonal dystrophy (INAD). INAD typically presents in the first year of life, and leads to optic atrophy and psychomotor regression. We have examined PLA2G6 expression in early human embryonic development by in situ hybridization. At Carnegie Stage (CS) 19 (approximately 7 post-conception weeks [PCW]), strong expression is evident in the ventricular zone (VZ) of midbrain and forebrain suggestive of expression in neural stem and progenitor cells. At CS23 (8PCW) expression is also detectable in the VZ of the hindbrain and the subventricular zone (SVZ) of the developing neocortex, ganglionic eminences and diencephalon. By 9PCW strong expression in the post-mitotic cells of the cortical plate can be seen in the developing neocortex. In the eye, expression is seen in the lens and retina at all stages examined. PLA2G6 expression is also evident in the alar plate of the spinal cord, dorsal root ganglia, the retina and lens in the eye and several non-neuronal tissues, including developing bones, lung, kidney and gut. These findings suggest a role for PLA2G6 in neuronal proliferation throughout the developing brain and in maturing neurons in the cortical plate and hindbrain. Although widespread PLA2G6 expression is detected in neuronal tissues, the pattern shows dynamic changes with time and indicates that INAD pathogenesis may begin prior to birth.