PLD1 promotes dendritic spine development by inhibiting ADAM10-mediated N-cadherin cleavage

Phospholipase D1 produces phosphatidic acid at sites of secretory vesicle docking and fusion

Phospholipase D1 (PLD1) activity is essential for the stimulated exocytosis of secretory vesicles where it acts as a lipid-modifying enzyme to produces phosphatidic acid (PA). PLD1 localizes to the plasma membrane and secretory vesicles, and PLD1 inhibition or knockdowns reduce the rate of fusion. However, temporal data resolving when and where PLD1 and PA are required during exocytosis is lacking. In this work, PLD1 and production of PA are measured during the trafficking, docking, and fusion of secretory vesicles in PC12 cells. Using fluorescently tagged PLD1 and a PA-binding protein, cells were imaged using TIRF microscopy to monitor the presence of PLD1 and the formation of PA throughout the stages of exocytosis. Single docking and fusion events were imaged to measure the recruitment of PLD1 and the formation of PA. PLD1 is present on mobile, docking, and fusing vesicles and also colocalizes with Syx1a clusters. Treatment of cells with PLD inhibitors significantly reduces fusion, but not PLD1 localization to secretory vesicles. Inhibitors also alter the formation of PA; when PLD1 is active, PA slowly accumulates on docked vesicles. During fusion, PA is reduced in cells treated with PLD1 inhibitors, indicating that PLD1 produces PA during exocytosis.

Phospholipase D1 promotes astrocytic differentiation through the FAK/AURKA/STAT3 signaling pathway in hippocampal neural stem/progenitor cells

Phospholipase D1 (PLD1) plays a crucial role in cell differentiation of different cell types. However, the involvement of PLD1 in astrocytic differentiation remains uncertain. In the present study, we investigate the possible role of PLD1 and its product phosphatidic acid (PA) in astrocytic differentiation of hippocampal neural stem/progenitor cells (NSPCs) from hippocampi of embryonic day 16.5 rat embryos. We showed that overexpression of PLD1 increased the expression level of glial fibrillary acidic protein (GFAP), an astrocyte marker, and the number of GFAP-positive cells. Knockdown of PLD1 by transfection with Pld1 shRNA inhibited astrocytic differentiation. Moreover, PLD1 deletion (Pld1^-/-) suppressed the level of GFAP in the mouse hippocampus. These results indicate that PLD1 plays a crucial role in regulating astrocytic differentiation in hippocampal NSPCs. Interestingly, PA itself was sufficient to promote astrocytic differentiation. PA-induced GFAP expression was decreased by inhibition of signal transducer and activation of transcription 3 (STAT3) using siRNA. Furthermore, PA-induced STAT3 activation and astrocytic differentiation were regulated by the focal adhesion kinase (FAK)/aurora kinase A (AURKA) pathway. Taken together, our findings suggest that PLD1 is an important modulator of astrocytic differentiation in hippocampal NSPCs via the FAK/AURKA/STAT3 signaling pathway.

Flying under the radar: CDH2 (N-cadherin), an important hub molecule in neurodevelopmental and neurodegenerative diseases

CDH2 belongs to the classic cadherin family of Ca²⁺-dependent cell adhesion molecules with a meticulously described dual role in cell adhesion and β-catenin signaling. During CNS development, CDH2 is involved in a wide range of processes including maintenance of neuroepithelial integrity, neural tube closure (neurulation), confinement of radial glia progenitor cells (RGPCs) to the ventricular zone and maintaining their proliferation-differentiation balance, postmitotic neural precursor migration, axon guidance, synaptic development and maintenance. In the past few years, direct and indirect evidence linked CDH2 to various neurological diseases, and in this review, we summarize recent developments regarding CDH2 function and its involvement in pathological alterations of the CNS.

Exome sequencing and analysis of 454,787 UK Biobank participants

A major goal in human genetics is to use natural variation to understand the phenotypic consequences of altering each protein-coding gene in the genome. Here we used exome sequencing¹ to explore protein-altering variants and their consequences in 454,787 participants in the UK Biobank study². We identified 12 million coding variants, including around 1 million loss-of-function and around 1.8 million deleterious missense variants. When these were tested for association with 3,994 health-related traits, we found 564 genes with trait associations at P ≤ 2.18 × 10^-11. Rare variant associations were enriched in loci from genome-wide association studies (GWAS), but most (91%) were independent of common variant signals. We discovered several risk-increasing associations with traits related to liver disease, eye disease and cancer, among others, as well as risk-lowering associations for hypertension (SLC9A3R2), diabetes (MAP3K15, FAM234A) and asthma (SLC27A3). Six genes were associated with brain imaging phenotypes, including two involved in neural development (GBE1, PLD1). Of the signals available and powered for replication in an independent cohort, 81% were confirmed; furthermore, association signals were generally consistent across individuals of European, Asian and African ancestry. We illustrate the ability of exome sequencing to identify gene-trait associations, elucidate gene function and pinpoint effector genes that underlie GWAS signals at scale.

Enhancing α-secretase Processing for Alzheimer's Disease-A View on SFRP1

Amyloid β (Aβ) peptides generated via sequential β- and γ-secretase processing of the amyloid precursor protein (APP) are major etiopathological agents of Alzheimer's disease (AD). However, an initial APP cleavage by an α-secretase, such as the a disintegrin and metalloproteinase domain-containing protein ADAM10, precludes β-secretase cleavage and leads to APP processing that does not produce Aβ. The latter appears to underlie the disease symptom-attenuating effects of a multitude of experimental therapeutics in AD animal models. Recent work has indicated that an endogenous inhibitor of ADAM10, secreted-frizzled-related protein 1 (SFRP1), is elevated in human AD brains and associated with amyloid plaques in mouse AD models. Importantly, genetic or functional attenuation of SFRP1 lowered Aβ accumulation and improved AD-related histopathological and neurological traits. Given SFRP1's well-known activity in attenuating Wnt signaling, which is also commonly impaired in AD, SFRP1 appears to be a promising therapeutic target for AD. This idea, however, needs to be addressed with care because of cancer enhancement potentials resulting from a systemic loss of SFRP1 activity, as well as an upregulation of ADAM10 activity. In this focused review, I shall discuss α-secretase-effected APP processing in AD with a focus on SFRP1, and explore the contrasting perspectives arising from the recent findings.

Phospholipase D as a key modulator of cancer progression

The phospholipase D (PLD) family has a ubiquitous expression in cells. PLD isoforms (PLDs) and their hydrolysate phosphatidic acid (PA) have been demonstrated to engage in multiple stages of cancer progression. Aberrant expression of PLDs, especially PLD1 and PLD2, has been detected in various cancers. Inhibition or elimination of PLDs activity has been shown to reduce tumour growth and metastasis. PLDs and PA also serve as downstream effectors of various cell-surface receptors, to trigger and regulate propagation of intracellular signals in the process of tumourigenesis and metastasis. Here, we discuss recent advances in understanding the functions of PLDs and PA in discrete stages of cancer progression, including cancer cell growth, invasion and migration, and angiogenesis, with special emphasis on the tumour-associated signalling pathways mediated by PLDs and PA and the functional importance of PLDs and PA in cancer therapy.

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.

PLD1 promotes dendritic spine morphogenesis via activating PKD1

Dendritic spines on the dendrites of pyramidal neurons are one of the most important components for excitatory synapses, where excitatory information exchanges and integrates. The defects of dendritic spine development have been closely connected with many nervous system diseases including autism, intellectual disability and so forth. Based on our previous studies, we here report a new functional signaling link between phospholipase D1 (PLD1) and protein kinase D1 (PKD1) in dendritic spine morphogenesis. Coimmunoprecipitation assays showed that PLD1 associates with PKD1. A series of knocking down and rescuing experiments demonstrated that PLD1 acts upstream of PKD1 in positively regulating dendritic spine morphogenesis. Using PLD1 inhibitor, we found that PLD1 activates PKD1 to promote dendritic spine morphogenesis. Thus, we further reveal the roles of the two different enzymes in neuronal development.

Stage-dependent involvement of ADAM10 and its significance in epileptic seizures

The prevalence of epileptic seizures in Alzheimer's disease (AD) has attracted an increasing amount of attention in recent years, and many cohort studies have found several risk factors associated with the genesis of seizures in AD. Among these factors, young age and severe dementia are seemingly contradictory and independent risk factors, indicating that the pathogenesis of epileptic seizures is, to a certain extent, stage‐dependent. A disintegrin and metalloproteinase domain‐containing protein 10 (ADAM10) is a crucial α‐secretase responsible for ectodomain shedding of its substrates; thus, the function of this protein depends on the biological effects of its substrates. Intriguingly, transgenic models have demonstrated ADAM10 to be associated with epilepsy. Based on the biological effects of its substrates, the potential pathogenic roles of ADAM10 in epileptic seizures can be classified into amyloidogenic processes in the ageing stage and cortical dysplasia in the developmental stage. Therefore, ADAM10 is reviewed here as a stage‐dependent modulator in the pathogenesis of epilepsy. Current data regarding ADAM10 in epileptic seizures were collected and reviewed for potential pathogenic roles (ie amyloidogenic processes and cortical dysplasia) and regulatory mechanisms (ie transcriptional and posttranscriptional regulation). These findings are then discussed in terms of the significance of the stage‐dependent functions of ADAM10 in epilepsy. Several potential targets for seizure control, such as candidate transcription factors and microRNAs that regulate ADAM10, as well as potential genetic screening tools for the early recognition of cortical dysplasia, have been suggested but must be studied in more detail.

Phospholipase D1 Signaling: Essential Roles in Neural Stem Cell Differentiation

Phospholipase D1 (PLD1) is generally accepted as playing an important role in the regulation of multiple cell functions, such as cell growth, survival, differentiation, membrane trafficking, and cytoskeletal organization. Recent findings suggest that PLD1 also plays an important role in the regulation of neuronal differentiation of neuronal cells. Moreover, PLD1-mediated signaling molecules dynamically regulate the neuronal differentiation of neural stem cells (NSCs). Rho family GTPases and Ca²⁺-dependent signaling, in particular, are closely involved in PLD1-mediated neuronal differentiation of NSCs. Moreover, PLD1 has a significant effect on the neurogenesis of NSCs via the regulation of SHP-1/STAT3 activation. Therefore, PLD1 has now attracted significant attention as an essential neuronal signaling molecule in the nervous system. In the current review, we summarize recent findings on the regulation of PLD1 in neuronal differentiation and discuss the potential role of PLD1 in the neurogenesis of NSCs.

Elevated phospholipase D isoform 1 in Alzheimer's disease patients' hippocampus: Relevance to synaptic dysfunction and memory deficits

Introduction

Phospholipase D (PLD), a lipolytic enzyme that breaks down membrane phospholipids, is also involved in signaling mechanisms downstream of seven transmembrane receptors. Abnormally elevated levels of PLD activity are well-established in Alzheimer's disease (AD), implicating the two isoforms of mammalian phosphatidylcholine cleaving PLD (PC-PLD1 and PC-PLD2). Therefore, we took a systematic approach of investigating isoform-specific expression in human synaptosomes and further investigated the possibility of therapeutic intervention using preclinical studies.

Methods

Synaptosomal Western blot analyses on the postmortem human hippocampus, temporal cortex, and frontal cortex of AD patient brains/age-matched controls and the 3XTg-AD mice hippocampus (mouse model with overexpression of human amyloid precursor protein, presenilin-1 gene, and microtubule-associated protein tau causing neuropathology progressing comparable to that in human AD patients) were used to detect the levels of neuronal PLD1 expression. Mouse hippocampal long-term potentiation of PLD1-dependent changes was studied using pharmacological approaches in ex vivo slice preparations from wild-type and transgenic mouse models. Finally, PLD1-dependent changes in novel object recognition memory were assessed following PLD1 inhibition.

Results

We observed elevated synaptosomal PLD1 in the hippocampus/temporal cortex from postmortem tissues of AD patients compared to age-matched controls and age-dependent hippocampal PLD1 increases in 3XTg-AD mice. PLD1 inhibition blocked effects of oligomeric amyloid β or toxic oligomeric tau species on high-frequency stimulation long-term potentiation and novel object recognition deficits in wild-type mice. Finally, PLD1 inhibition blocked long-term potentiation deficits normally observed in aging 3XTg-AD mice.

Discussion

Using human studies, we propose a novel role for PLD1-dependent signaling as a critical mechanism underlying oligomer-driven synaptic dysfunction and consequent memory disruption in AD. We, further, provide the first set of preclinical studies toward future therapeutics targeting PLD1 in slowing down/stopping the progression of AD-related memory deficits as a complementary approach to immunoscavenging clinical trials that are currently in progress.