Deficiency of Calcium-Independent Phospholipase A2 Beta Induces Brain Iron Accumulation through Upregulation of Divalent Metal Transporter 1

Unveiling the role of iPLA2β in neurodegeneration: From molecular mechanisms to advanced therapies

Calcium-independent phospholipase A2β (iPLA2β), a member of the phospholipase A2 (PLA2s) superfamily, is encoded by the PLA2G6 gene. Mutations in the PLA2G6 gene have been identified as the primary cause of infantile neuroaxonal dystrophy (INAD) and, less commonly, as a contributor to Parkinson's disease (PD). Recent studies have revealed that iPLA2β deficiency leads to neuroinflammation, iron accumulation, mitochondrial dysfunction, lipid dysregulation, and other pathological changes, forming a complex pathogenic network. These discoveries shed light on potential mechanisms underlying PLA2G6-associated neurodegeneration (PLAN) and offer valuable insights for therapeutic development. This review provides a comprehensive analysis of the fundamental characteristics of iPLA2β, its association with neurodegeneration, the pathogenic mechanisms involved in PLAN, and potential targets for therapeutic intervention. It offers an overview of the latest advancements in this field, aiming to contribute to ongoing research endeavors and facilitate the development of effective therapies for PLAN.

Emerging roles of ferroptosis in infectious diseases

In living organisms, lipid peroxidation is a continuously occurring cellular process and therefore involved in various physiological and pathological contexts. Among the broad variety of lipids, polyunsaturated fatty acids (PUFA) constitute a major target of oxygenation either when released as mediators by phospholipases or when present in membranous phospholipids. The last decade has seen the characterization of an iron- and lipid peroxidation-dependent cell necrosis, namely, ferroptosis, that involves the accumulation of peroxidized PUFA-containing phospholipids. Further studies could link ferroptosis in a very large body of (physio)-pathological processes, including cancer, neurodegenerative, and metabolic diseases. In this review, we mostly focus on the emerging involvement of lipid peroxidation-driven ferroptosis in infectious diseases, and the immune consequences. We also discuss the putative ability of microbial virulence factors to exploit or to dampen ferroptosis regulatory pathways to their own benefit.

Vicious cycle of lipid peroxidation and iron accumulation in neurodegeneration

Lipid peroxidation and iron accumulation are closely associated with neurodegenerative diseases, such as Alzheimer's, Parkinson's, and Huntington's diseases, or neurodegeneration with brain iron accumulation disorders. Mitochondrial dysfunction, lipofuscin accumulation, autophagy disruption, and ferroptosis have been implicated as the critical pathomechanisms of lipid peroxidation and iron accumulation in these disorders. Currently, the connection between lipid peroxidation and iron accumulation and the initial cause or consequence in neurodegeneration processes is unclear. In this review, we have compiled the known mechanisms by which lipid peroxidation triggers iron accumulation and lipofuscin formation, and the effect of iron overload on lipid peroxidation and cellular function. The vicious cycle established between both pathological alterations may lead to the development of neurodegeneration. Therefore, the investigation of these mechanisms is essential for exploring therapeutic strategies to restrict neurodegeneration. In addition, we discuss the interplay between lipid peroxidation and iron accumulation in neurodegeneration, particularly in PLA2G6-associated neurodegeneration, a rare neurodegenerative disease with autosomal recessive inheritance, which belongs to the group of neurodegeneration with brain iron accumulation disorders.

PPARδ Activation Mitigates 6-OHDA-Induced Neuronal Damage by Regulating Intracellular Iron Levels

Intracellular iron accumulation in dopaminergic neurons contributes to neuronal cell death in progressive neurodegenerative disorders such as Parkinson’s disease. However, the mechanisms of iron homeostasis in this context remain incompletely understood. In the present study, we assessed the role of the nuclear receptor peroxisome proliferator-activated receptor δ (PPARδ) in cellular iron homeostasis. We identified that PPARδ inhibited 6-hydroxydopamine (6-OHDA)-triggered neurotoxicity in SH-SY5Y neuroblastoma cells. PPARδ activation with GW501516, a specific PPARδ agonist, mitigated 6-OHDA-induced neuronal damage. Further, PPARδ activation also suppressed iron accumulation, which contributes to 6-OHDA-induced neuronal damage. PPARδ activation attenuated 6-OHDA-induced neuronal damage in a similar manner to that of the iron chelator deferoxamine. We further elucidated that PPARδ modulated cellular iron homeostasis by regulating expression of divalent metal transporter 1, ferroportin 1, and ferritin, but not transferrin receptor 1, through iron regulatory protein 1 in 6-OHDA-treated cells. Interestingly, PPARδ activation suppressed 6-OHDA-triggered generation of reactive oxygen species and lipid peroxides. The effects of GW501516 were abrogated by shRNA knockdown of PPARδ, indicating that the effects of GW501516 were PPARδ-dependent. Taken together, these findings suggest that PPARδ attenuates 6-OHDA-induced neurotoxicity by preventing intracellular iron accumulation, thereby suppressing iron overload-associated generation of reactive oxygen species and lipid peroxides, key mediators of ferroptotic cell death.

PLA2G6 Silencing Suppresses Melanoma Progression and Affects Ferroptosis Revealed by Quantitative Proteomics

Although phospholipase A2 group VI (PLA2G6) is involved in oncogenesis in several human tumors, its expression and role in cutaneous malignant melanoma (CMM) pathogenesis remains unclear. Here, by using the Oncomine and CCLE online database, immunohistochemistry, RT-qPCR, and western blotting analysis, we revealed that PLA2G6 was markedly up-regulated in CMM tissues compared to nevus tissues, as well as remarkably increased in vitro in SK-MEL-28 and M14 melanoma cell lines compared to human melanocytes. In vivo, PLA2G6 was also elevated in nine melanoma tissues compared to adjacent tissues. To investigate the malignant behaviors of PLA2G6 in CMM, SK-MEL-28 and M14 cell lines with PLA2G6 stable knockdown by RNAi strategy were constructed. Through CCK8 and colony formation assays in vitro and xenograft tumor experiment in vivo, we found that knockdown of PLA2G6 dramatically inhibited cell proliferation. The results of scratch-wound and transwell assays suggested that the migration and invasion of melanoma cells were prominently suppressed after silencing PLA2G6. In addition, flow cytometry showed that the knockdown of PLA2G6 promoted the apoptosis rate of melanoma cells. To further explore the potential molecular mechanism, we used liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) proteomic and bioinformatics analysis. The GO and KEGG analysis suggested that the underlying mechanism of PLA2G6 in CMM might be associated with the ferroptosis pathway, and ferroptosis-related proteins were validated to be differentially expressed in PLA2G6 knockdown SK-MEL-28 and M14 cells. Together, these results suggested that PLA2G6 knockdown significantly inhibited cell proliferation, metastasis, and promoted apoptosis in melanoma. Our findings on the biological function of PLA2G6 and the underlying association between PLA2G6 and ferroptosis in melanoma may contribute to developing a potential therapeutic strategy for melanoma.

WDR45, one gene associated with multiple neurodevelopmental disorders

The WDR45 gene is localized on the X-chromosome and variants in this gene are linked to six different neurodegenerative disorders, i.e., ß-propeller protein associated neurodegeneration, Rett-like syndrome, intellectual disability, and epileptic encephalopathies including developmental and epileptic encephalopathy, early-onset epileptic encephalopathy and West syndrome and potentially also specific malignancies. WDR45/WIPI4 is a WD-repeat β-propeller protein that belongs to the WIPI (WD repeat domain, phosphoinositide interacting) family. The precise cellular function of WDR45 is still largely unknown, but deletions or conventional variants in WDR45 can lead to macroautophagy/autophagy defects, malfunctioning mitochondria, endoplasmic reticulum stress and unbalanced iron homeostasis, suggesting that this protein functions in one or more pathways regulating directly or indirectly those processes. As a result, the underlying cause of the WDR45-associated disorders remains unknown. In this review, we summarize the current knowledge about the cellular and physiological functions of WDR45 and highlight how genetic variants in its encoding gene may contribute to the pathophysiology of the associated diseases. In particular, we connect clinical manifestations of the disorders with their potential cellular origin of malfunctioning and critically discuss whether it is possible that one of the most prominent shared features, i.e., brain iron accumulation, is the primary cause for those disorders.

Abbreviations: ATG/Atg: autophagy related; BPAN: ß-propeller protein associated neurodegeneration; CNS: central nervous system; DEE: developmental and epileptic encephalopathy; EEG: electroencephalograph; ENO2/neuron-specific enolase, enolase 2; EOEE: early-onset epileptic encephalopathy; ER: endoplasmic reticulum; ID: intellectual disability; IDR: intrinsically disordered region; MRI: magnetic resonance imaging; NBIA: neurodegeneration with brain iron accumulation; NCOA4: nuclear receptor coactivator 4; PtdIns3P: phosphatidylinositol-3-phosphate; RLS: Rett-like syndrome; WDR45: WD repeat domain 45; WIPI: WD repeat domain, phosphoinositide interacting

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.

Genetic perspective on the synergistic connection between vesicular transport, lysosomal and mitochondrial pathways associated with Parkinson's disease pathogenesis

Parkinson's disease (PD) and atypical parkinsonian syndromes (APS) are symptomatically characterized by parkinsonism, with the latter presenting additionally a distinctive range of atypical features. Although the majority of patients with PD and APS appear to be sporadic, genetic causes of several rare monogenic disease variants were identified. The knowledge acquired from these genetic factors indicated that defects in vesicular transport pathways, endo-lysosomal dysfunction, impaired autophagy-lysosomal protein and organelle degradation pathways, α-synuclein aggregation and mitochondrial dysfunction play key roles in PD pathogenesis. Moreover, membrane dynamics are increasingly recognized as a key player in the disease pathogenesis due lipid homeostasis alterations, associated with lysosomal dysfunction, caused by mutations in several PD and APS genes. The importance of lysosomal dysfunction and lipid homeostasis is strengthened by both genetic discoveries and clinical epidemiology of the association between parkinsonism and lysosomal storage disorders (LSDs), caused by the disruption of lysosomal biogenesis or function. A synergistic coordination between vesicular trafficking, lysosomal and mitochondria defects exist whereby mutations in PD and APS genes encoding proteins primarily involved one PD pathway are frequently associated with defects in other PD pathways as a secondary effect. Moreover, accumulating clinical and genetic observations suggest more complex inheritance patters of familial PD exist, including oligogenic and polygenic inheritance of genes in the same or interconnected PD pathways, further strengthening their synergistic connection.Here, we provide a comprehensive overview of PD and APS genes with functions in vesicular transport, lysosomal and mitochondrial pathways, and highlight functional and genetic evidence of the synergistic connection between these PD associated pathways.

Parkinson's disease and iron

While the initial causes of Parkinson's disease (PD) are not clearly defined, iron deposition has long been implicated in the pathogenesis of PD. The substantia nigra of PD patients, where the selective loss of dopaminergic neurons occurs, show a fairly selective and significant elevation in iron contents. However, the question remains whether iron deposition represents the initiation cause or merely the consequence of nigral degeneration. Here, we describe existing findings regarding the interaction of iron with neuromelanin and alpha synuclein, the iron deposition in experimental animal model of PD and sporadic and familial PD patients, and the treatment option involving the use of iron chelators for targeting the aberration of iron level in brain. This review may provide us a better understanding of the role of iron in PD to address the question of cause or consequence.

Deferiprone and efonidipine mitigated iron-overload induced neurotoxicity in wild-type and thalassemic mice

AIMS

We previously demonstrated that iron-overload in non-thalassemic rats induced neurotoxicity and cognitive decline. However, the effect of iron-overload on the brain of thalassemic condition has never been investigated. An iron chelator (deferiprone) provides neuroprotective effects against metal toxicity. Furthermore, a T-type calcium channels blocker (efonidipine) effectively attenuates cardiac dysfunction in thalassemic mice with iron-overload. However, the effects of both drugs on brain of iron-overload thalassemia has not been determined. We hypothesize that iron-overload induces neurotoxicity in Thalassemic and wild-type mice, and not only deferiprone, but also efonidipine, provides neuroprotection against iron-overload condition.

MAIN METHODS

Mice from both wild-type (WT) and β-thalassemic type (HT) groups were assigned to be fed with a standard-diet or high-iron diet containing 0.2% ferrocene/kg of diet (HFe) for 4 months consecutively. After three months of HFe, 75-mg/kg/d deferiprone or 4-mg/kg/d efonidipine were administered to the HFe-fed WT and HT mice for 1 month.

KEY FINDINGS

HFe consumption caused an equal impact on circulating iron-overload, oxidative stress, and inflammation in WT and HT mice. Brain iron-overload and iron-mediated neurotoxicity, such as oxidative stress, inflammation, glial activation, mitochondrial dysfunction, and Alzheimer's like pathologies, were observed to an equal degree in HFe fed WT and HT mice. These pathological conditions were mitigated by both deferiprone and efonidipine.

SIGNIFICANCE

These findings indicate that iron-overload itself caused neurotoxicity, and T-type calcium channels may play a role in this condition.

DMT1 Expression and Iron Levels at the Crossroads Between Aging and Neurodegeneration

Iron homeostasis is an essential prerequisite for metabolic and neurological functions throughout the healthy human life, with a dynamic interplay between intracellular and systemic iron metabolism. The development of different neurodegenerative diseases is associated with alterations of the intracellular transport of iron and heavy metals, principally mediated by Divalent Metal Transporter 1 (DMT1), responsible for Non-Transferrin Bound Iron transport (NTBI). In addition, DMT1 regulation and its compartmentalization in specific brain regions play important roles during aging. This review highlights the contribution of DMT1 to the physiological exchange and distribution of body iron and heavy metals during aging and neurodegenerative diseases. DMT1 also mediates the crosstalk between central nervous system and peripheral tissues, by systemic diffusion through the Blood Brain Barrier (BBB), with the involvement of peripheral iron homeostasis in association with inflammation. In conclusion, a survey about the role of DMT1 and iron will illustrate the complex panel of interrelationship with aging, neurodegeneration and neuroinflammation.

Parkinson Disease from Mendelian Forms to Genetic Susceptibility: New Molecular Insights into the Neurodegeneration Process

Parkinson disease (PD) is known as a common progressive neurodegenerative disease which is clinically diagnosed by the manifestation of numerous motor and nonmotor symptoms. PD is a genetically heterogeneous disorder with both familial and sporadic forms. To date, researches in the field of Parkinsonism have identified 23 genes or loci linked to rare monogenic familial forms of PD with Mendelian inheritance. Biochemical studies revealed that the products of these genes usually play key roles in the proper protein and mitochondrial quality control processes, as well as synaptic transmission and vesicular recycling pathways within neurons. Despite this, large number of patients affected with PD typically tends to show sporadic forms of disease with lack of a clear family history. Recent genome-wide association studies (GWAS) meta-analyses on the large sporadic PD case-control samples from European populations have identified over 12 genetic risk factors. However, the genetic etiology that underlies pathogenesis of PD is also discussed, since it remains unidentified in 40% of all PD-affected cases. Nowadays, with the emergence of new genetic techniques, international PD genomics consortiums and public online resources such as PDGene, there are many hopes that future large-scale genetics projects provide further insights into the genetic etiology of PD and improve diagnostic accuracy and therapeutic clinical trial designs.

Ferrous Iron Up-regulation in Fibroblasts of Patients with Beta Propeller Protein-Associated Neurodegeneration (BPAN)

Mutations in WDR45 gene, coding for a beta-propeller protein, have been found in patients affected by Neurodegeneration with Brain Iron Accumulation, NBIA5 (also known as BPAN). BPAN is a movement disorder with Non Transferrin Bound Iron (NTBI) accumulation in the basal ganglia as common hallmark between NBIA classes (Hayflick et al., 2013). WDR45 has been predicted to have a role in autophagy, while the impairment of iron metabolism in the different NBIA subclasses has not currently been clarified. We found the up-regulation of the ferrous iron transporter (-)IRE/Divalent Metal Transporter1 and down-regulation of Transferrin receptor in the fibroblasts of two BPAN affected patients with splicing mutations 235+1G>A (BPAN1) and 517_519ΔVal 173 (BPAN2). The BPAN patients showed a concomitant increase of intracellular ferrous iron after starvation. An altered pattern of iron transporters with iron overload is highlighted in BPAN human fibroblasts, supporting for a role of DMT1 in NBIA. We here present a novel element, about iron accumulation, to the existing knowledge in field of NBIA. Attention is focused to a starvation-dependent iron overload, possibly accounting for iron accumulation in the basal ganglia. Further investigation could clarify iron regulation in BPAN.

Progressive Axonal Degeneration of Nigrostriatal Dopaminergic Neurons in Calcium-Independent Phospholipase A2β Knockout Mice

Calcium-independent phospholipase A₂β (iPLA₂β, PLA2G6) is essential for the remodeling of membrane glycerophospholipids. Mutations in this gene are responsible for autosomal recessive, young onset, L-dopa-responsive parkinsonism (PARK14), suggesting a neurodegenerative condition in the nigrostriatal dopaminergic system in patients with PLA2G6 mutations. We previously observed slowly progressive motor deficits in iPLA₂β-knockout (KO) mice. To clarify whether a deficiency of iPLA₂β leads to the degeneration of nigrostriatal dopaminergic neurons, we analyzed the striatum of iPLA₂β-KO mice. At all clinical stages, nerve terminals in the striatum were immunopositive for tyrosine hydroxylase (TH) and dopamine transporter (DAT) in wild-type (WT) control mice. In iPLA₂β-KO mice, focal loss of nerve terminals positive for TH and DAT was found from 56 weeks (early clinical stage), although iPLA₂β-KO mice at 56 weeks showed no significant decrease in the number of dopaminergic neurons in the substantia nigra compared with age-matched WT mice, as reported previously. At 100 weeks (late clinical stage), greater decreases in DAT immunoreactivity were observed in the striatum of iPLA₂β-KO mice. Moreover, strongly TH-positive structures, presumed to be deformed axons, were observed in the neuropils of the striatum of iPLA₂β-KO mice starting at 15 weeks (preclinical stage) and increased with age. These results suggest that the degeneration of dopaminergic neurons occurs mainly in the distal region of axons in iPLA₂β-KO mice.

High expression of α-synuclein in damaged mitochondria with PLA2G6 dysfunction

To clarify the role of α-synuclein (αSyn) in neuronal membrane remodeling, we analyzed the expression of αSyn in neurons with a dysfunction of PLA2G6, which is indispensable for membrane remodeling. αSyn/phosphorylated-αSyn (PαSyn) distribution and neurodegeneration were quantitatively estimated in PLA2G6-knockout (KO) mice, which demonstrate marked mitochondrial membrane degeneration. We also assessed the relationship between αSyn deposits and mitochondria in brain tissue from patients with PLA2G6-associated neurodegeneration (PLAN) and Parkinson’s disease (PD), and quantitatively examined Lewy bodies (LBs) and neurons. The expression level of αSyn was elevated in PLA2G6-knockdown cells and KO mouse neurons. Strong PαSyn expression was observed in neuronal granules in KO mice before onset of motor symptoms. The granules were mitochondrial outer membrane protein (TOM20)-positive. Ultramicroscopy revealed that PαSyn-positive granules were localized to mitochondria with degenerated inner membranes. After symptom onset, TOM20-positive granules were frequently found in ubiquitinated spheroids, where PαSyn expression was low. Axons were atrophic, but the neuronal loss was not evident in KO mice. In PLAN neurons, small PαSyn-positive inclusions with a TOM20-positive edge were frequently observed and clustered into LBs. The surfaces of most LBs were TOM20-positive in PLAN and TOM20-negative in PD brains. The high proportion of LB-bearing neurons and the preserved neuronal number in PLAN suggested long-term survival of LB-bearing neurons. Elevated expression of αSyn/PαSyn in mitochondria appears to be the early response to PLA2G6-deficiency in neurons. The strong affinity of αSyn for damaged mitochondrial membranes may promote membrane stabilization of mitochondria and neuronal survival in neurons.