Reciprocal regulation of very low density lipoprotein receptors (VLDLRs) in neurons by brain-derived neurotrophic factor (BDNF) and Reelin: involvement of the E3 ligase Mylip/Idol

Targeting NPC1 in Renal Cell Carcinoma

Rapidly proliferating cancer cells have a greater requirement for cholesterol than normal cells. Tumor cells are largely dependent on exogenous lipids given that their growth requirements are not fully met by endogenous pathways. Our current study shows that ccRCC cells have redundant mechanisms of cholesterol acquisition. We demonstrate that all major lipoproteins (i.e., LDL, HDL, and VLDL) have a comparable ability to support the growth of ccRCC cells and are equally effective in counteracting the antitumor activities of TKIs. The intracellular trafficking of exogenous lipoprotein-derived cholesterol appears to be distinct from the movement of endogenously synthesized cholesterol. De novo synthetized cholesterol is transported from the endoplasmic reticulum directly to the plasma membrane and to the acyl-CoA: cholesterol acyltransferase, whereas lipoprotein-derived cholesterol is distributed through the NPC1-dependent endosomal trafficking system. Expression of NPC1 is increased in ccRCC at mRNA and protein levels, and high expression of NPC1 is associated with poor prognosis. Our current findings show that ccRCC cells are particularly sensitive to the inhibition of endolysosomal cholesterol export and underline the therapeutic potential of targeting NPC1 in ccRCC.

Identification of the molecular subgroups in Alzheimer's disease by transcriptomic data

Background

Alzheimer's disease (AD) is a heterogeneous pathological disease with genetic background accompanied by aging. This inconsistency is present among molecular subtypes, which has led to diagnostic ambiguity and failure in drug development. We precisely distinguished patients of AD at the transcriptome level.

Methods

We collected 1,240 AD brain tissue samples collected from the GEO dataset. Consensus clustering was used to identify molecular subtypes, and the clinical characteristics were focused on. To reveal transcriptome differences among subgroups, we certificated specific upregulated genes and annotated the biological function. According to RANK METRIC SCORE in GSEA, TOP10 was defined as the hub gene. In addition, the systematic correlation between the hub gene and “A/T/N” was analyzed. Finally, we used external data sets to verify the diagnostic value of hub genes.

Results

We identified three molecular subtypes of AD from 743 AD samples, among which subtypes I and III had high-risk factors, and subtype II had protective factors. All three subgroups had higher neuritis plaque density, and subgroups I and III had higher clinical dementia scores and neurofibrillary tangles than subgroup II. Our results confirmed a positive association between neurofibrillary tangles and dementia, but not neuritis plaques. Subgroup I genes clustered in viral infection, hypoxia injury, and angiogenesis. Subgroup II showed heterogeneity in synaptic pathology, and we found several essential beneficial synaptic proteins. Due to presenilin one amplification, Subgroup III was a risk subgroup suspected of familial AD, involving abnormal neurogenic signals, glial cell differentiation, and proliferation. Among the three subgroups, the highest combined diagnostic value of the hub genes were 0.95, 0.92, and 0.83, respectively, indicating that the hub genes had sound typing and diagnostic ability.

Conclusion

The transcriptome classification of AD cases played out the pathological heterogeneity of different subgroups. It throws daylight on the personalized diagnosis and treatment of AD.

Effects of statins on the inducible degrader of low-density lipoprotein receptor in familial hypercholesterolemia

The inducible degrader of low-density lipoprotein receptor (IDOL) is an E3 ubiquitin ligase involved in the post-transcriptional regulation of LDL receptor (LDLR). Statins lower plasma LDL by activating transcription of hepatic LDLR expression, and we have determined whether statins modulate IDOL expression and influence LDLR protein abundance. IDOL expression in monocytes and serum IDOL level was determined in statin-treated familial hypercholesterolemia (FH) patients and compared with control subjects. Serum IDOL level was also evaluated in a group of untreated FH patients before and after the initiation of statin. The mechanism underlying the inhibitory effect of statin on IDOL expression was investigated in vitro. In statin-treated FH patients, serum IDOL level and its expression in monocytes was reduced compared with control (P < 0.05). In contrast, untreated FH patients had higher serum levels of IDOL and proprotein convertase subtilisin/kexintype 9 (PCSK9) than control (P < 0.05), and serum IDOL level decreased after statin therapy (P < 0.05) whereas an increase was observed in PCSK9 level (P < 0.01). In vitro, atorvastatin significantly decreased IDOL abundance in a dose-dependent manner in cultured macrophages and hepatocytes with a concomitant increase in LDLR expression. The transcription of IDOL was restored by adding either an LXR agonist T0901317 or oxysterol 22(R)-hydroxycholesterol, indicating that statin inhibited IDOL expression by reducing LXR activation. The LXR-IDOL-LDLR axis can be modulated by statins in vitro and in vivo. Statins inhibit IDOL expression by reducing LXR activation and upregulate LDLR, and statins exert the opposite effect on IDOL and PCSK9.

The Role of MeCP2 in Regulating Synaptic Plasticity in the Context of Stress and Depression

Methyl-CpG-binding protein 2 (MeCP2) is a transcriptional regulator that is highly abundant in the brain. It binds to methylated genomic DNA to regulate a range of physiological functions implicated in neuronal development and adult synaptic plasticity. MeCP2 has mainly been studied for its role in neurodevelopmental disorders, but alterations in MeCP2 are also present in stress-related disorders such as major depression. Impairments in both stress regulation and synaptic plasticity are associated with depression, but the specific mechanisms underlying these changes have not been identified. Here, we review the interplay between stress, synaptic plasticity, and MeCP2. We focus our attention on the transcriptional regulation of important neuronal plasticity genes such as BDNF and reelin (RELN). Moreover, we provide evidence from recent studies showing a link between chronic stress-induced depressive symptoms and dysregulation of MeCP2 expression, underscoring the role of this protein in stress-related pathology. We conclude that MeCP2 is a promising target for the development of novel, more efficacious therapeutics for the treatment of stress-related disorders such as depression.

PGC-1α Signaling Increases GABA(A) Receptor Subunit α2 Expression, GABAergic Neurotransmission and Anxiety-Like Behavior in Mice

Peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) is a master regulator of mitochondria biogenesis and cell stress playing a role in metabolic and degenerative diseases. In the brain PGC-1α expression has been localized mainly to GABAergic interneurons but its overall role is not fully understood. We observed here that the protein levels of γ-aminobutyric acid (GABA) type A receptor-α2 subunit (GABARα2) were increased in hippocampus and brain cortex in transgenic (Tg) mice overexpressing PGC-1α in neurons. Along with this, GABARα2 expression was enhanced in the hippocampus of the PGC-1α Tg mice, as shown by quantitative PCR. Double immunostaining revealed that GABARα2 co-localized with the synaptic protein gephyrin in higher amounts in the striatum radiatum layer of the hippocampal CA1 region in the Tg compared with Wt mice. Electrophysiology revealed that the frequency of spontaneous and miniature inhibitory postsynaptic currents (mIPSCs) was increased in the CA1 region in the Tg mice, indicative of an augmented GABAergic transmission. Behavioral tests revealed an increase for anxiety-like behavior in the PGC-1α Tg mice compared with controls. To study whether drugs acting on PPARγ can affect GABARα2, we employed pioglitazone that elevated GABARα2 expression in primary cultured neurons. Similar results were obtained using the specific PPARγ agonist, N-(2-benzoylphenyl)-O-[2-(methyl-2-pyridinylamino) ethyl]-L-tyrosine hydrate (GW1929). These results demonstrate that PGC-1α regulates GABARα2 subunits and GABAergic neurotransmission in the hippocampus with behavioral consequences. This indicates further that drugs like pioglitazone, widely used in the treatment of type 2 diabetes, can influence GABARα2 expression via the PPARγ/PGC-1α system.

Regulation of Pain Genes-Capsaicin vs Resiniferatoxin: Reassessment of Transcriptomic Data

Emerging evidence has shown a strong association between neuropathic pain and chronic diseases. In recent years, the treatment of neuropathic pain has attracted more attention. Natural products, such as capsaicin and resiniferatoxin, have been well utilized to treat this disease. In this study, we aim to compare the regulatory effects of capsaicin and resiniferatoxin on pain-related genes as well as on genes with no direct association with pain. Public transcriptomic and microarray data on gene expression in the dorsal root ganglia and genes associated with TRPV1 (+) neurons were obtained from the GEO database and then analyzed. Differentially expressed genes were selected for further functional analysis, including pathway enrichment, protein-protein interaction, and regulatory network analysis. Pain-associated genes were extracted with the reference of two pain gene databases and the effects of these two natural drugs on the pain-associated genes were measured. The results of our research indicate that as compared to capsaicin, resiniferatoxin (RTX) regulates more non pain-associated genes and has a negative impact on beneficial genes (off-targets) which are supposed to alleviate nociception and hypersensitivity by themselves. So, based on this study, we may conclude that capsaicin may be less potent when compared to RTX, but it will elicit considerably less adverse effects too. Thereby confirming that capsaicin could be used for the efficient alleviation of neuropathic pain with possibly fewer side effects.

Purification of an Intact Human Protein Overexpressed from Its Endogenous Locus via Direct Genome Engineering

The overproduction and purification of human proteins is a requisite of both basic and medical research. Although many recombinant human proteins have been purified, current protein production methods have several limitations; recombinant proteins are frequently truncated, fail to fold properly, and/or lack appropriate post-translational modifications. In addition, such methods require subcloning of the target gene into relevant plasmids, which can be difficult for long proteins with repeated domains. Here we devised a novel method for target protein production by introduction of a strong promoter for overexpression and an epitope tag for purification in front of the endogenous human gene, in a sense performing molecular cloning directly in the human genome, which does not require cloning of the target gene. As a proof of concept, we successfully purified intact human Reelin protein, which is lengthy (3460 amino acids) and contains repeating domains, and confirmed that it was biologically functional.

VLDL and HDL attenuate endoplasmic reticulum and metabolic stress in HL-1 cardiomyocytes

Lipoproteins have a vital role in the development of metabolic and cardiovascular diseases ranging from protective to deleterious effects on target tissues. VLDL has been shown to induce lipotoxic lipid accumulation and exert a variety of negative effects on cardiomyocytes. Lipotoxicity and endoplasmic reticulum (ER) stress are proposed to be the mediators of damaging effects of metabolic diseases on cardiovascular system. We treated cardiomyocytes with lipoproteins to evaluate the adaptability of these cells to metabolic stress induced by starvation and excess of lipoproteins, and to evaluate the effect of lipoproteins and lipid accumulation on ER stress. VLDL reversed metabolic stress induced by starvation, while HDL did not. VLDL induced dose-dependent lipid accumulation in cardiomyocytes, which however did not result in reduced cell viability or induction of ER stress. Moreover, VLDL or HDL pre-treatment reduced ER stress in cardiomyocytes induced by tunicamycin and palmitic acid as measured by the expression of ER stress markers, even in conditions of increased lipid accumulation. VLDL and HDL induced activation of pro-survival ERK1/2 in cardiomyocytes; however, this activation was not involved in the protection against ER stress. Additionally, we observed that LDLR and VLDLR are regulated differently by lipoproteins and cellular stress, as lipoproteins induced VLDLR protein independently of the level of lipid accumulation. We conclude that VLDL is not a priori detrimental for cardiomyocytes and can even have beneficial effects, enabling cell survival under starvation and attenuating ER stress.

Dynamic Interaction of USP14 with the Chaperone HSC70 Mediates Crosstalk between the Proteasome, ER Signaling, and Autophagy

USP14 is a deubiquitinating enzyme associated with the proteasome important for protein degradation. Here we show that upon proteasome inhibition or expression of the mutant W58A-USP14, association of USP14 with the 19S regulatory particle is disrupted. MS-based interactomics revealed an interaction of USP14 with the chaperone, HSC70, in neuroblastoma cells. Proteasome inhibition enhanced binding of USP14 to HSC70, and to XBP1u and IRE1α proteins, demonstrating a role in the unfolded protein response. Striatal neurons expressing mutant huntingtin exhibited reduced USP14 and HSC70 levels, whereas inhibition of HSC70 downregulated USP14. Furthermore, proteasome inhibition or use of the mutant W58A-USP14 facilitated the interaction of USP14 with the autophagy protein, GABARAP. Functionally, overexpression of W58A-USP14 increased GABARAP positive autophagosomes in striatal neurons, and this was abrogated using the HSC70 inhibitor, VER-155008. Modulation of the USP14-HSC70 axis may represent a potential therapeutic target in HD to beneficially influence multiple proteostasis pathways.

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USP14 binds HSC70 upon proteasome inhibition

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This rises GABARAP autophagosomes in HD

Caspase-2 and p75 neurotrophin receptor (p75NTR) are involved in the regulation of SREBP and lipid genes in hepatocyte cells

Lipid-induced toxicity is part of several human diseases, but the mechanisms involved are not fully understood. Fatty liver is characterized by the expression of different growth and tissue factors. The neurotrophin, nerve growth factor (NGF) and its pro-form, pro-NGF, are present in fatty liver together with p75 neurotrophin receptor (p75NTR). Stimulation of human Huh7 hepatocyte cells with NGF and pro-NGF induced Sterol-regulator-element-binding protein-2 (SREBP2) activation and increased Low-Density Lipoprotein Receptor (LDLR) expression. We observed that phosphorylation of caspase-2 by p38 MAPK was essential for this regulation involving a caspase-3-mediated cleavage of SREBP2. RNA sequencing showed that several genes involved in lipid metabolism were altered in p75NTR-deficient mouse liver. The same lipogenic genes were downregulated in p75NTR gene-engineered human Huh7 cells and reciprocally upregulated by stimulation of p75NTRs. In the knock-out mice the serum cholesterol and triglyceride levels were reduced, suggesting a physiological role of p75NTRs in whole-body lipid metabolism. Taken together, this study shows that p75NTR signaling influences a network of genes involved in lipid metabolism in liver and hepatocyte cells. Modulation of p75NTR signaling may be a target to consider in various metabolic disorders accompanied by increased lipid accumulation.

The E3 ubiquitin ligase inducible degrader of the LDL receptor/myosin light chain interacting protein in health and disease

PURPOSE OF REVIEW

The RING E3 ubiquitin ligase inducible degrader of the LDL receptor (IDOL, also known as MYLIP) promotes ubiquitylation and subsequent lysosomal degradation of the LDL receptor (LDLR), thus acting to limit uptake of lipoprotein-derived cholesterol into cells. Next to the LDLR, IDOL also promotes degradation of two related receptors, the very LDL receptor (VLDLR) and apolipoprotein E receptor 2 (APOER2), which have important signaling functions in the brain. We review here the emerging role of IDOL in lipoprotein and energy metabolism, neurodegenerative diseases, and the potential for therapeutic targeting of IDOL.

RECENT FINDINGS

Genetic studies suggest an association between IDOL and lipoprotein metabolism in humans. Studies in rodents and nonhuman primates support an in-vivo role for IDOL in lipoprotein metabolism, and also uncovered an unexpected role in whole-body energy metabolism. Recent evaluation of IDOL function in the brain revealed a role in memory formation and progression of Alzheimer's disease. The report of the first IDOL inhibitor may facilitate further investigations on therapeutic strategies to target IDOL.

SUMMARY

IDOL is emerging as an important determinant of lipid and energy metabolism in metabolic disease as well as in Alzheimer's disease. IDOL targeting may be beneficial in treating these conditions.

LXR Regulation of Brain Cholesterol: From Development to Disease

Liver X receptors (LXRs) are master regulators of cholesterol homeostasis and inflammation in the central nervous system (CNS). The brain, which contains a disproportionately large amount of the body's total cholesterol (∼25%), requires a complex and delicately balanced cholesterol metabolism to maintain neuronal function. Dysregulation of cholesterol metabolism has been implicated in numerous neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD). Due to their cholesterol-sensing and anti-inflammatory activities, LXRs are positioned centrally in the everyday maintenance of CNS function. This review focuses on recent research into the role of LXRs in the CNS during normal development and homeostasis and in disease states.

p75 Neurotrophin Receptor Signaling Activates Sterol Regulatory Element-binding Protein-2 in Hepatocyte Cells via p38 Mitogen-activated Protein Kinase and Caspase-3

Nerve growth factor (NGF) influences the survival and differentiation of a specific population of neurons during development, but its role in non-neuronal cells has been less studied. We observed here that NGF and its pro-form, pro-NGF, are elevated in fatty livers from leptin-deficient mice compared with controls, concomitant with an increase in low density lipoprotein receptors (LDLRs). Stimulation of mouse primary hepatocytes with NGF or pro-NGF increased LDLR expression through the p75 neurotrophin receptor (p75NTR). Studies using Huh7 human hepatocyte cells showed that the neurotrophins activate the sterol regulatory element-binding protein-2 (SREBP2) that regulates genes involved in lipid metabolism. The mechanisms for this were related to stimulation of p38 mitogen-activated protein kinase (p38 MAPK) and activation of caspase-3 and SREBP2 cleavage following NGF and pro-NGF stimulations. Cell fractionation experiments showed that caspase-3 activity was increased particularly in the membrane fraction that harbors SREBP2 and caspase-2. Experiments showed further that caspase-2 interacts with pro-caspase-3 and that p38 MAPK reduced this interaction and caused caspase-3 activation. Because of the increased caspase-3 activity, the cells did not undergo cell death following p75NTR stimulation, possibly due to concomitant activation of nuclear factor-κB (NF-κB) pathway by the neurotrophins. These results identify a novel signaling pathway triggered by ligand-activated p75NTR that via p38 MAPK and caspase-3 mediate the activation of SREBP2. This pathway may regulate LDLRs and lipid uptake particularly after injury or during tissue inflammation accompanied by an increased production of growth factors, including NGF and pro-NGF.

Stress-induced upregulation of VLDL receptor alters Wnt-signaling in neurons

Lipoprotein receptor family members hold multiple roles in the brain, and alterations in lipoprotein receptor expression and function are implicated in neuronal stress, developmental disorders and neurodegenerative diseases, such as Alzheimer's disease. Berberine (BBR), a nutraceutical shown to have both neuroprotective and neurotoxic properties, is suggested to regulate lipoprotein receptor expression. We show that subtoxic concentration of BBR regulates neuronal lipoprotein receptor expression in a receptor- and time-dependent fashion in cerebellar granule neurons (CGN). Similarly to BBR, subtoxic concentrations of neuronal stressors cobalt chloride, thapsigargin and rotenone increased very-low-density lipoprotein receptor (VLDLR) mRNA and protein expression in CGN suggesting a conserved pathway for stress-induced upregulation of VLDLR in neurons. We also show that VLDLR upregulation is accompanied by transiently increased stabilization of hypoxia-induced factor 1 alpha (HIF-1α) and decreased β-catenin levels affecting the Wnt pathway through GSK3β phosphorylation, a crucial player in neurodegenerative processes. Our results indicate that neuronal stress differentially regulates lipoprotein receptor expression in neurons, with VLDLR upregulation as a common element as a modulator of neuronal Wnt signaling.

Nerve growth factor (NGF) and pro-NGF increase low-density lipoprotein (LDL) receptors in neuronal cells partly by different mechanisms: role of LDL in neurite outgrowth

Low-density lipoprotein receptors (LDLRs) mediate the uptake of lipoprotein particles into cells, as studied mainly in peripheral tissues. Here, we show that nerve growth factor (NGF) increases LDLR levels in PC6.3 cells and in cultured septal neurons from embryonic rat brain. Study of the mechanisms showed that NGF enhanced transcription of the LDLR gene, acting mainly via Tropomyosin receptor kinase A receptors. Simvastatin, a cholesterol-lowering drug, also increased the LDLR expression in PC6.3 cells. In addition, pro-NGF and pro-brain-derived neurotrophic factor, acting via the p75 neurotrophin receptor (p75NTR) also increased LDLRs. We further observed that Myosin Regulatory Light Chain-Interacting Protein/Inducible Degrader of the LDLR (Mylip/Idol) was down-regulated by pro-NGF, whereas the other LDLR regulator, proprotein convertase subtilisin kexin 9 (PCSK9) was not significantly changed. On the functional side, NGF and pro-NGF increased lipoprotein uptake by neuronal cells as shown using diacetyl-labeled LDL. The addition of serum-derived lipoprotein particles in conjunction with NGF or simvastatin enhanced neurite outgrowth. Collectively, these results show that NGF and simvastatin are able to stimulate lipoprotein uptake by neurons with a positive effect on neurite outgrowth. Increases in LDLRs and lipoprotein particles in neurons could play a functional role during brain development, in neuroregeneration and after brain injuries. Nerve growth factor (NGF) and pro-NGF induce the expression of low-density lipoprotein receptors (LDLRs) in neuronal cells leading to increased LDLR levels. Pro-NGF also down-regulated myosin regulatory light chain-interacting protein/inducible degrader of the LDLR (Mylip/Idol) that is involved in the degradation of LDLRs. NGF acts mainly via Tropomyosin receptor kinase A (TrkA) receptors, whereas pro-NGF stimulates p75 neurotrophin receptor (p75NTR). Elevated LDLRs upon NGF and pro-NGF treatments enhanced lipoprotein uptake by neurons. Addition of LDL particles further led to the stimulation of neurite outgrowth in PC6.3 cells after NGF or simvastatin treatments, suggesting a stimulatory role of lipoproteins on neuronal differentiation. In contrast, pro-NGF had no effect on neurite outgrowth either in the absence or presence of LDL particles. The precise mechanisms by which increased lipoproteins uptake can affect neurite outgrowth warrant further studies.

PCSK9 is present in human cerebrospinal fluid and is maintained at remarkably constant concentrations throughout the course of the day

Proprotein convertase subtilisin kexin type 9 (PCSK9) is a key regulator of serum low density lipoprotein cholesterol levels. PCSK9 is secreted by the liver and binds the hepatic low density lipoprotein receptor, causing its subsequent degradation. PCSK9 has also been shown to regulate the levels of additional membrane-bound proteins in vitro, including very low-density lipoprotein receptor, apolipoprotein E receptor 2, and beta-site amyloid precursor protein-cleaving enzyme 1, which are highly expressed in central nervous system (CNS) and have been implicated in Alzheimer’s disease. Previous studies have demonstrated that human circulating PCSK9 displays a diurnal rhythm. Currently, little is known about PCSK9 levels in human cerebrospinal fluid (CSF). In the present study, we measured PCSK9 concentrations in both serum and CSF collected from healthy human subjects at multiple time points throughout the day. While PCSK9 in serum manifested a distinct diurnal pattern, CSF PCSK9 levels were remarkably constant throughout the course of the day and were also consistently lower than corresponding serum PCSK9 concentrations. Our results indicate that regulation of PCSK9 in human CSF may be different than for plasma PCSK9, suggesting that further study of the role of PCSK9 in the CNS is warranted.

Neurotrophins regulate ApoER2 proteolysis through activation of the Trk signaling pathway

Background

ApoER2 and the neurotrophin receptors Trk and p75^NTR are expressed in the CNS and regulate key functional aspects of neurons, including development, survival, and neuronal function. It is known that both ApoER2 and p75^NTR are processed by metalloproteinases, followed by regulated intramembrane proteolysis. TrkA activation by nerve growth factor (NGF) increases the proteolytic processing of p75^NTR mediated by ADAM17. Reelin induces the sheeding of ApoER2 ectodomain depending on metalloproteinase activity. However, it is not known if there is a common regulation mechanism for processing these receptors.

Results

We found that TrkA activation by NGF in PC12 cells induced ApoER2 processing, which was dependent on TrkA activation and metalloproteinases. NGF-induced ApoER2 proteolysis was independent of mitogen activated protein kinase activity and of phosphatidylinositol-3 kinase activity. In contrast, the basal proteolysis of ApoER2 increased when both kinases were pharmacologically inhibited. The ApoER2 ligand reelin regulated the proteolytic processing of its own receptor but not of p75^NTR. Finally, in primary cortical neurons, which express both ApoER2 and TrkB, we found that the proteolysis of ApoER2 was also regulated by brain-derived growth factor (BDNF).

Conclusions

Our results highlight a novel relationship between neurotrophins and the reelin-ApoER2 system, suggesting that these two pathways might be linked to regulate brain development, neuronal survival, and some pathological conditions.