Increased expression of phospholipase D1 in the brains of scrapie-infected mice

Phospholipase D1 Attenuation Therapeutics Promotes Resilience against Synaptotoxicity in 12-Month-Old 3xTg-AD Mouse Model of Progressive Neurodegeneration

Abrogating synaptotoxicity in age-related neurodegenerative disorders is an extremely promising area of research with significant neurotherapeutic implications in tauopathies including Alzheimer's disease (AD). Our studies using human clinical samples and mouse models demonstrated that aberrantly elevated phospholipase D1 (PLD1) is associated with amyloid beta (Aβ) and tau-driven synaptic dysfunction and underlying memory deficits. While knocking out the lipolytic PLD1 gene is not detrimental to survival across species, elevated expression is implicated in cancer, cardiovascular conditions and neuropathologies, leading to the successful development of well-tolerated mammalian PLD isoform-specific small molecule inhibitors. Here, we address the importance of PLD1 attenuation, achieved using repeated 1 mg/kg of VU0155069 (VU01) intraperitoneally every alternate day for a month in 3xTg-AD mice beginning only from ~11 months of age (with greater influence of tau-driven insults) compared to age-matched vehicle (0.9% saline)-injected siblings. A multimodal approach involving behavior, electrophysiology and biochemistry corroborate the impact of this pre-clinical therapeutic intervention. VU01 proved efficacious in preventing in later stage AD-like cognitive decline affecting perirhinal cortex-, hippocampal- and amygdala-dependent behaviors. Glutamate-dependent HFS-LTP and LFS-LTD improved. Dendritic spine morphology showed the preservation of mushroom and filamentous spine characteristics. Differential PLD1 immunofluorescence and co-localization with Aβ were noted.

Role of Phospholipase D-Derived Phosphatidic Acid in Regulated Exocytosis and Neurological Disease

Lipids play a vital role in numerous cellular functions starting from a structural role as major constituents of membranes to acting as signaling intracellular or extracellular entities. Accordingly, it has been known for decades that lipids, especially those coming from diet, are important to maintain normal physiological functions and good health. On the other side, the exact molecular nature of these beneficial or deleterious lipids, as well as their precise mode of action, is only starting to be unraveled. This recent improvement in our knowledge is largely resulting from novel pharmacological, molecular, cellular, and genetic tools to study lipids in vitro and in vivo. Among these important lipids, phosphatidic acid plays a unique and central role in a great variety of cellular functions. This review will focus on the proposed functions of phosphatidic acid generated by phospholipase D in the last steps of regulated exocytosis with a specific emphasis on hormonal and neurotransmitter release and its potential impact on different neurological diseases.

Suppressing aberrant phospholipase D1 signaling in 3xTg Alzheimer's disease mouse model promotes synaptic resilience

Current approaches in treatment of Alzheimer's disease (AD) is focused on early stages of cognitive decline. Identifying therapeutic targets that promote synaptic resilience during early stages may prevent progressive memory deficits by preserving memory mechanisms. We recently reported that the inducible isoform of phospholipase D (PLD1) was significantly increased in synaptosomes from post-mortem AD brains compared to age-matched controls. Using mouse models, we reported that the aberrantly elevated neuronal PLD1 is key for oligomeric amyloid driven synaptic dysfunction and underlying memory deficits. Here, we demonstrate that chronic inhibition using a well-tolerated PLD1 specific small molecule inhibitor is sufficient to prevent the progression of synaptic dysfunction during early stages in the 3xTg-AD mouse model. Firstly, we report prevention of cognitive decline in the inhibitor-treated group using novel object recognition (NOR) and fear conditioning (FC). Secondly, we provide electrophysiological assessment of better synaptic function in the inhibitor-treated group. Lastly, using Golgi staining, we report that preservation of dendritic spine integrity as one of the mechanisms underlying the action of the small molecule inhibitor. Collectively, these studies provide evidence for inhibition of PLD1 as a potential therapeutic strategy in preventing progression of cognitive decline associated with AD and related dementia.

p62-Keap1-NRF2-ARE Pathway: A Contentious Player for Selective Targeting of Autophagy, Oxidative Stress and Mitochondrial Dysfunction in Prion Diseases

Prion diseases are a group of fatal and debilitating neurodegenerative diseases affecting humans and animal species. The conversion of a non-pathogenic normal cellular protein (PrP^c) into an abnormal infectious, protease-resistant, pathogenic form prion protein scrapie (PrP^Sc), is considered the etiology of these diseases. PrP^Sc accumulates in the affected individual's brain in the form of extracellular plaques. The molecular pathways leading to neuronal cell death in prion diseases are still unclear. The free radical damage, oxidative stress and mitochondrial dysfunction play a key role in the pathogenesis of the various neurodegenerative disorders including prion diseases. The brain is very sensitive to changes in the redox status. It has been demonstrated that PrP^c behaves as an antioxidant, while the neurotoxic prion peptide PrP^Sc increases hydrogen peroxide toxicity in the neuronal cultures leading to mitochondrial dysfunction and cell death. The nuclear factor erythroid 2-related factor 2 (NRF2) is an oxidative responsive pathway and a guardian of lifespan, which protect the cells from free radical stress-mediated cell death. The reduced glutathione, a major small molecule antioxidant present in all mammalian cells, and produced by several downstream target genes of NRF2, counterbalances the mitochondrial reactive oxygen species (ROS) production. In recent years, it has emerged that the ubiquitin-binding protein, p62-mediated induction of autophagy, is crucial for NRF2 activation and elimination of mitochondrial dysfunction and oxidative stress. The current review article, focuses on the role of NRF2 pathway in prion diseases to mitigate the disease progression.

p62-Keap1-NRF2-ARE Pathway: A Contentious Player for Selective Targeting of Autophagy, Oxidative Stress and Mitochondrial Dysfunction in Prion Diseases

Prion diseases are a group of fatal and debilitating neurodegenerative diseases affecting humans and animal species. The conversion of a non-pathogenic normal cellular protein (PrP^c) into an abnormal infectious, protease-resistant, pathogenic form prion protein scrapie (PrP^Sc), is considered the etiology of these diseases. PrP^Sc accumulates in the affected individual’s brain in the form of extracellular plaques. The molecular pathways leading to neuronal cell death in prion diseases are still unclear. The free radical damage, oxidative stress and mitochondrial dysfunction play a key role in the pathogenesis of the various neurodegenerative disorders including prion diseases. The brain is very sensitive to changes in the redox status. It has been demonstrated that PrP^c behaves as an antioxidant, while the neurotoxic prion peptide PrP^Sc increases hydrogen peroxide toxicity in the neuronal cultures leading to mitochondrial dysfunction and cell death. The nuclear factor erythroid 2-related factor 2 (NRF2) is an oxidative responsive pathway and a guardian of lifespan, which protect the cells from free radical stress-mediated cell death. The reduced glutathione, a major small molecule antioxidant present in all mammalian cells, and produced by several downstream target genes of NRF2, counterbalances the mitochondrial reactive oxygen species (ROS) production. In recent years, it has emerged that the ubiquitin-binding protein, p62-mediated induction of autophagy, is crucial for NRF2 activation and elimination of mitochondrial dysfunction and oxidative stress. The current review article, focuses on the role of NRF2 pathway in prion diseases to mitigate the disease progression.

Phosphatidylinositol-glycan-phospholipase D is involved in neurodegeneration in prion disease

PrP^Sc is formed from a normal glycosylphosphatidylinositol (GPI)-anchored prion protein (PrP^C) by a posttranslational modification. Most GPI-anchored proteins have been shown to be cleaved by GPI phospholipases. Recently, GPI-phospholipase D (GPI-PLD) was shown to be a strictly specific enzyme for GPI anchors. To investigate the involvement of GPI-PLD in the processes of neurodegeneration in prion diseases, we examined the mRNA and protein expression levels of GPI-PLD in the brains of a prion animal model (scrapie), and in both the brains and cerebrospinal fluids (CSF) of sporadic and familial Creutzfeldt-Jakob disease (CJD) patients. We found that compared with controls, the expression of GPI-PLD was dramatically down-regulated in the brains of scrapie-infected mice, especially in the caveolin-enriched membrane fractions. Interestingly, the observed decrease in GPI-PLD expression levels began at the same time that PrP^Sc began to accumulate in the infected brains and this decrease was also observed in both the brain and CSF of CJD patients; however, no differences in expression were observed in either the brains or CSF specimens from Alzheimer’s disease patients. Taken together, these results suggest that the down-regulation of GPI-PLD protein may be involved in prion propagation in the brains of prion diseases.

Association of phosphatidic acid with the bovine mitochondrial ADP/ATP carrier

The beef heart adenine nucleotide carrier protein (Anc) of the inner mitochondrial membrane can be purified in a form stabilized by binding the inhibitor carboxyatractyloside. The protein is copurified with bound lipid. We show for the first time that phosphatidic acid, although a minor component, is one of the lipids bound to Anc. The short spin-lattice relaxation time found by (31)P magic angle spinning nuclear magnetic resonance (MAS/NMR) for phosphatidic acid indicates that it is tightly bound to the protein. However, this lipid also has a comparatively small chemical shift anisotropy, suggesting that it can undergo rapid reorientation in space. In contrast, most of the lipid bound to Anc shows anisotropic motion typical of a bilayer arrangement. The phosphatidic acid that is detected in the purified preparation of Anc is also shown to be present initially in the unfractionated mitochondria, prior to the isolation of Anc. In Triton-solubilized mitochondria, phosphatidic acid, cardiolipin, phosphatidylethanolamine, and phosphatidylcholine exhibit resonance lines in the static (31)P NMR spectra, but in the purified Anc, only the phosphatidylethanolamine and phosphatidylcholine can be detected by this method, even though the other lipids are still present. This demonstrates that the phosphatidic acid and cardiolipin are interacting with the Anc. The thermal denaturation of the Anc was determined by differential scanning calorimetry. The protein denatures at 74 degrees C both before and after the NMR studies with the same characteristics.

Internalization and signal transduction of PrP(106-126) in neuronal cells

Invasion of the nervous system and neuronal spread of infection are critical, but poorly understood steps in the pathogenesis of prion diseases. We have thus analyzed the internalization and signal transduction of the neurotoxic fragment of the prion protein PrP(106-126) in the rat neuroblastoma cell line B104 by fluorescence microscopy and quantification by ELISA and in primary neuronal cells from mice. Phospholipase D (PLD) is known to be an enzyme involved in the regulation of secretion, endocytosis and receptor signalling. We determined the PLD activity using a transphosphatidylation assay and could show that PLD is involved in PrP(106-126) internalization. The determination of receptor activity via quantification of ERK1/2 phosphorylation and cAMP level measurement verified the PrP(106-126)-induced signal transduction in B104 cells and primary neuronal cells. PrP(106-126)-induced a decrease in cAMP level in neuronal cells. These studies indicate the involvement of PLD in PrP(106-126)-endocytosis and mediated cellular signalling by an unidentified inhibitory G-protein-coupled receptor and may allow the development of therapeutic agents interfering with prion uptake and/or PLD function using PLD as a possible pharmaceutical target.

Accumulation of citrullinated proteins by up-regulated peptidylarginine deiminase 2 in brains of scrapie-infected mice: a possible role in pathogenesis

Peptidylarginine deiminases (PADs), which are a group of posttranslational modification enzymes, are involved in protein citrullination (deimination) by the conversion of peptidylarginine to peptidylcitrulline in a calcium concentration-dependent manner. Among the PADs, PAD2 is widely distributed in various tissues and is the only type that is expressed in brain. To elucidate the involvement of protein citrullination by PAD2 in the pathogenesis of brain-specific prion diseases, we examined the profiles of citrullinated proteins using the brains of scrapie-infected mice as a prion disease model. We found that, compared with controls, increased levels of citrullinated proteins of various molecular weights were detected in different brain sections of scrapie-infected mice. In support of this data, expression levels of PAD2 protein as well as its enzyme activity were significantly increased in brain sections of scrapie-infected mice, including hippocampus, brain stem, and striatum. Additionally, the expression levels of PAD2 mRNA were increased during scrapie infection. Moreover, PAD2 immunoreactivity was increased in scrapie-infected brains, with staining detected primarily in reactive astrocytes. Using two-dimensional electrophoresis and matrix-assisted laser desorption/ionization-time of flight mass spectrometry, various citrullinated proteins were identified in the brains of scrapie-infected mice, including glial fibrillary acidic protein, myelin basic protein, enolases, and aldolases. This study suggests that accumulated citrullinated proteins and abnormal activation of PAD2 may function in the pathogenesis of prion diseases and serve as potential therapeutic targets.

Ischemic preconditioning upregulates expression of phospholipase D2 in the rat hippocampus

To investigate the possible involvement of phospholipase D2 (PLD2) in the induction of ischemic tolerance, we analyzed the distribution and time course of PLD2 expression in the rat hippocampus after a sublethal period of ischemia. Forebrain ischemia was induced by four-vessel occlusion for 3 min. Increased PLD2 immunoreactivity after this sublethal ischemia was observed in CA1 pyramidal neurons of the rat hippocampus. In tolerance-acquired CA1 neurons, PLD2 immunoreactivity was upregulated as early as 12 h post-ischemia and was most prominent at 1-3 days, with expression sustained for at least 7 days, as shown by a time course of immunoblotting and measurement of the enzymatic activity of PLD. PLD2 expression was also increased in ischemia-resistant CA3 neurons and dentate granule cells, although weaker staining intensity was noted. Further, we showed that, in cultured SK-N-BE(2)C human neuroblastoma cells, overexpression of PLD2 inhibited cell death by chemical hypoxia induced with potassium cyanide and deoxyglucose. These data suggest that upregulation of PLD2 might be involved in the neuroprotective mechanism of ischemic tolerance in the rat hippocampus.

Internalization of PrP106-126 by the formyl-peptide-receptor-like-1 in glial cells

Recent studies suggest that the formyl-peptide-receptor-like-1 (FPRL1) plays an essential role in inflammatory responses in the host defence mechanisms and neurodegenerative disorders. Furthermore, it may be involved in proinflammatory processes of prion diseases. However, little is known about the induction and regulation of PrP106-126-induced receptor endocytosis. We have thus analysed whether PrP106-126 increases the activity of phospholipase D (PLD) via FPRL1, an enzyme involved in the regulation of the secretion, endocytosis and receptor signalling, in glial cells. PLD activity was determined using a transphosphatidylation assay and the internalization of PrP106-126, and FPRL1 was assessed by fluorescence microscopy and quantified by ELISA. We could show that PLD is activated by PrP106-126 both in astrocytes and microglia, and moreover that PrP106-126 is rapidly internalized via FPRL1 in astrocytes and microglia cells. The determination of receptor activity by extracellular signal-regulated kinases 1/2 phosphorylation and cAMP level measurement verified the PrP106-126-induced activation of FPRL1. FPRL1-mediated PrP106-126 uptake was blocked by the receptor antagonist chenodeoxycholic acid. These studies indicate the involvement of FPRL1-mediated cellular signalling in PrP106-126-endocytosis and may allow the development of therapeutic agents interfering with prion uptake and/or PLD function, using either PLD or the FPRL1 as a possible pharmaceutical target.

Phospholipase D1 is up-regulated in the mitochondrial fraction from the brains of Alzheimer's disease patients

Mitochondrial dysfunction may play an important role in sporadic Alzheimer's disease (AD) progression. Recently, we have reported that amyloid precursor protein (APP) stimulates phospholipase D (PLD) activity and beta-amyloid (Abeta) region of APP is involved in the interaction with PLD1. To elucidate the involvement of PLD in the pathophysiology of AD, we examined the expression of PLD1 and alteration of membrane phospholipid in mitochondrial membranes of control and AD brains using Western blot and phospholipid analysis by thin layer chromatography (TLC). We have found that protein expression of PLD1 was significantly increased in mitochondrial fraction of brains of AD patients compared with that in control brains. Furthermore, the concentration of mitochondrial phospholipids such as phosphatidylcholine (PC) and phosphatidylethanolamine (PE) was increased and the content of phosphatidic acid (PA), a product of PLD activity, was up-regulated in the mitochondrial membrane fractions of AD brain compared with that of control brain. These results suggest that up-regulation of PLD1 in the mitochondrial fraction of AD brain might affect the composition of membrane phospholipids and provide a clue to the mechanism underlying the mitochondrial dysfunction associated with AD.

Phospholipase D1 is associated with amyloid precursor protein in Alzheimer's disease

Amyloid precursor protein (APP) is a widely expressed transmembrane protein of unknown function that is involved in the pathogenesis of Alzheimer's disease (AD). We investigated the involvement of phospholipase D (PLD) in the pathophysiology of AD. We showed dramatic upregulation of PLD1 immunoreactivity in reactive astroglial cells in brain tissue sections from authentic AD patients. Expression and activity of PLD1 were up-regulated in brain tissues from AD patients, especially caveolae membrane fraction, compared with those of control brains. Interestingly, PLD1 physically interacts and colocalizes with APP and caveolin-3. We found that APP was associated with the pleckstrin homology domain of PLD1, and the amyloid region of APP interacted with PLD. Elevated expression of APP stimulated PLD activity in human astroglioma cells. These results suggest that up-regulation of PLD might have a role in the neuronal pathology associated with AD.

CDP-choline significantly restores phosphatidylcholine levels by differentially affecting phospholipase A2 and CTP: phosphocholine cytidylyltransferase after stroke

Phosphatidylcholine (PtdCho) is a major membrane phospholipid, and its loss is sufficient in itself to induce cell death. PtdCho homeostasis is regulated by the balance between hydrolysis and synthesis. PtdCho is hydrolyzed by phospholipase A2 (PLA2), PtdChospecific phospholipase C (PtdCho-PLC), and phospholipase D (PLD). PtdCho synthesis is rate-limited by CTP:phosphocholine cytidylyltransferase (CCT), which makes CDP-choline. The final step of PtdCho synthesis is catalyzed by CDP-choline:1,2-diacylglycerol cholinephosphotransferase. PtdCho synthesis in the brain is predominantly through the CDP-choline pathway. Transient middle cerebral artery occlusion (tMCAO) significantly increased PLA2 activity, secretory PLA2 (sPLA2)-IIA mRNA and protein levels, PtdCho-PLC activity, and PLD2 protein expression following reperfusion. CDP-choline treatment significantly attenuated PLA2 activity, sPLA2-IIA mRNA and protein levels, and PtdCho-PLC activity, but did not affect PLD2 protein expression. tMCAO also resulted in loss of CCT activity and CCTalpha protein, which were partially restored by CDP-choline. No changes were observed in cytosolic PLA2 or calcium-independent PLA2 tMCAO. protein levels after Up-regulation of PLA2, PtdCho-PLC, and PLD and regulation of CCT collectively down-resulted in loss of PtdCho, which was significantly restored by CDP-choline treatment. CDP-choline treatment significantly attenuated the infarction volume by 55 +/- 5% after 1 h of tMCAO and 1 day of reperfusion. Taken together, these results suggest that CDP-choline significantly restores Ptd-Cho levels by differentially affecting sPLA2-IIA, PtdCho-PLC, and CCTalpha after transient focal cerebral ischemia. A hypothetical scheme is proposed integrating results from this study and from other reports in the literature.

Functions and pathophysiological roles of phospholipase D in the brain

Ten years after the isoforms of mammalian phospholipase D (PLD), PLD1 and 2, were cloned, their roles in the brain remain speculative but several lines of evidence now implicate these enzymes in basic cell functions such as vesicular trafficking as well as in brain development. Many mitogenic factors, including neurotransmitters and growth factors, activate PLD in neurons and astrocytes. Activation of PLD downstream of protein kinase C seems to be a required step for astroglial proliferation. The characteristic disruption of the PLD signaling pathway by ethanol probably contributes to the delay of brain growth in fetal alcohol syndrome. The post-natal increase of PLD activities concurs with synapto- and myelinogenesis in the brain and PLD is apparently involved in neurite formation. In the adult and aging brain, PLD activity has antiapoptotic properties suppressing ceramide formation. Increased PLD activities in acute and chronic neurodegeneration as well as in inflammatory processes are evidently due to astrogliosis and may be associated with protective responses of tissue repair and remodeling. ARF-regulated PLD participates in receptor endocytosis as well as in exocytosis of neurotransmitters where PLD seems to favor vesicle fusion by modifications of the shape and charge of lipid membranes. Finally, PLD activities contribute free choline for the synthesis of acetylcholine in the brain. Novel tools such as RNA interference should help to further elucidate the roles of PLD isoforms in brain physiology and pathology.