Lipid raft localization and palmitoylation: Identification of two requirements for cell death induction by the tumor suppressors UNC5H

Netrin-1 in Post-stroke Neuroprotection: Beyond Axon Guidance Cue

BACKGROUND

Stroke, especially ischemic stroke, is a leading disease associated with death and long-term disability with limited therapeutic options. Neuronal death caused by vascular impairment, programmed cell death and neuroinflammation has been proven to be associated with increased stroke severity and poor stroke recovery. In light of this, a development of neuroprotective drugs targeting injured neurons is urgently needed for stroke treatment. Netrin-1, known as a bifunctional molecule, was originally described to mediate the repulsion or attraction of axonal growth by interacting with its different receptors. Importantly, accumulating evidence has shown that netrin-1 can manifest its beneficial functions to brain tissue repair and neural regeneration in different neurological disease models.

OBJECTIVE

In this review, we focus on the implications of netrin-1 and its possibly involved pathways on neuroprotection after ischemic stroke, through which a better understanding of the underlying mechanisms of netrin-1 may pave the way to novel treatments.

METHODS

Peer-reviewed literature was recruited by searching databases of PubMed, Scopus, Embase, and Web of Science till the year 2021.

CONCLUSION

There has been certain evidence to support the neuroprotective function of netrin-1 by regulating angiogenesis, autophagy, apoptosis and neuroinflammation after stroke. Netrin-1 may be a promising drug candidate in reducing stroke severity and improving outcomes.

One Raft to Guide Them All, and in Axon Regeneration Inhibit Them

Central nervous system damage caused by traumatic injuries, iatrogenicity due to surgical interventions, stroke and neurodegenerative diseases is one of the most prevalent reasons for physical disability worldwide. During development, axons must elongate from the neuronal cell body to contact their precise target cell and establish functional connections. However, the capacity of the adult nervous system to restore its functionality after injury is limited. Given the inefficacy of the nervous system to heal and regenerate after damage, new therapies are under investigation to enhance axonal regeneration. Axon guidance cues and receptors, as well as the molecular machinery activated after nervous system damage, are organized into lipid raft microdomains, a term typically used to describe nanoscale membrane domains enriched in cholesterol and glycosphingolipids that act as signaling platforms for certain transmembrane proteins. Here, we systematically review the most recent findings that link the stability of lipid rafts and their composition with the capacity of axons to regenerate and rebuild functional neural circuits after damage.

Growth cone repulsion to Netrin-1 depends on lipid raft microdomains enriched in UNC5 receptors

During brain development, Uncoordinated locomotion 5 (UNC5) receptors control axonal extension through their sensing of the guidance molecule Netrin-1. The correct positioning of receptors into cholesterol-enriched membrane raft microdomains is crucial for the efficient transduction of the recognized signals. However, whether such microdomains are required for the appropriate axonal guidance mediated by UNC5 receptors remains unknown. Here, we combine the use of confocal microscopy, live-cell FRAP analysis and single-particle tracking PALM to characterize the distribution of UNC5 receptors into raft microdomains, revealing differences in their membrane mobility properties. Using pharmacological and genetic approaches in primary neuronal cultures and brain cerebellar explants we further demonstrate that disrupting raft microdomains inhibits the chemorepulsive response of growth cones and axons against Netrin-1. Together, our findings indicate that the distribution of all UNC5 receptors into cholesterol-enriched raft microdomains is heterogeneous and that the specific localization has functional consequences for the axonal chemorepulsion against Netrin-1.

Mendelian randomization while jointly modeling cis genetics identifies causal relationships between gene expression and lipids

Inference of causality between gene expression and complex traits using Mendelian randomization (MR) is confounded by pleiotropy and linkage disequilibrium (LD) of gene-expression quantitative trait loci (eQTL). Here, we propose an MR method, MR-link, that accounts for unobserved pleiotropy and LD by leveraging information from individual-level data, even when only one eQTL variant is present. In simulations, MR-link shows false-positive rates close to expectation (median 0.05) and high power (up to 0.89), outperforming all other tested MR methods and coloc. Application of MR-link to low-density lipoprotein cholesterol (LDL-C) measurements in 12,449 individuals with expression and protein QTL summary statistics from blood and liver identifies 25 genes causally linked to LDL-C. These include the known SORT1 and ApoE genes as well as PVRL2, located in the APOE locus, for which a causal role in liver was not known. Our results showcase the strength of MR-link for transcriptome-wide causal inferences.

Mendelian randomization is a useful tool to infer causal relationships between traits, but can be confounded by the presence of pleiotropy. Here, the authors have developed MR-link, a Mendelian randomization method which accounts for unobserved pleiotropy and linkage disequilibrium between instrumental variables.

Progress toward Understanding Protein S-acylation: Prospective in Plants

S-acylation, also known as S-palmitoylation or palmitoylation, is a reversible post-translational lipid modification in which long chain fatty acid, usually the 16-carbon palmitate, covalently attaches to a cysteine residue(s) throughout the protein via a thioester bond. It is involved in an array of important biological processes during growth and development, reproduction and stress responses in plant. S-acylation is a ubiquitous mechanism in eukaryotes catalyzed by a family of enzymes called Protein S-Acyl Transferases (PATs). Since the discovery of the first PAT in yeast in 2002 research in S-acylation has accelerated in the mammalian system and followed by in plant. However, it is still a difficult field to study due to the large number of PATs and even larger number of putative S-acylated substrate proteins they modify in each genome. This is coupled with drawbacks in the techniques used to study S-acylation, leading to the slower progress in this field compared to protein phosphorylation, for example. In this review we will summarize the discoveries made so far based on knowledge learnt from the characterization of protein S-acyltransferases and the S-acylated proteins, the interaction mechanisms between PAT and its specific substrate protein(s) in yeast and mammals. Research in protein S-acylation and PATs in plants will also be covered although this area is currently less well studied in yeast and mammalian systems.

Lipid-Free Apolipoprotein A-I Reduces Progression of Atherosclerosis by Mobilizing Microdomain Cholesterol and Attenuating the Number of CD131 Expressing Cells: Monitoring Cholesterol Homeostasis Using the Cellular Ester to Total Cholesterol Ratio

BACKGROUND

Atherosclerosis is a chronic inflammatory disorder whose development is inversely correlated with high-density lipoprotein concentration. Current therapies involve pharmaceuticals that significantly elevate plasma high-density lipoprotein cholesterol concentrations. Our studies were conducted to investigate the effects of low-dose lipid-free apolipoprotein A-I (apoA-I) on chronic inflammation. The aims of these studies were to determine how subcutaneously injected lipid-free apoA-I reduces accumulation of lipid and immune cells within the aortic root of hypercholesterolemic mice without sustained elevations in plasma high-density lipoprotein cholesterol concentrations.

METHODS AND RESULTS

Ldlr^-/- and Ldlr^-/- apoA-I^-/- mice were fed a Western diet for a total of 12 weeks. After 6 weeks, a subset of mice from each group received subcutaneous injections of 200 μg of lipid-free human apoA-I 3 times a week, while the other subset received 200 μg of albumin, as a control. Mice treated with lipid-free apoA-I showed a decrease in cholesterol deposition and immune cell retention in the aortic root compared with albumin-treated mice, regardless of genotype. This reduction in atherosclerosis appeared to be directly related to a decrease in the number of CD131 expressing cells and the esterified cholesterol to total cholesterol content in several immune cell compartments. In addition, apoA-I treatment altered microdomain cholesterol composition that shifted CD131, the common β subunit of the interleukin 3 receptor, from lipid raft to nonraft fractions of the plasma membrane.

CONCLUSIONS

ApoA-I treatment reduced lipid and immune cell accumulation within the aortic root by systemically reducing microdomain cholesterol content in immune cells. These data suggest that lipid-free apoA-I mediates beneficial effects through attenuation of immune cell lipid raft cholesterol content, which affects numerous types of signal transduction pathways that rely on microdomain integrity for assembly and activation.

Protein S-palmitoylation and cancer

Protein S-palmitoylation is a reversible posttranslational modification of proteins with fatty acids, an enzymatic process driven by a recently discovered family of protein acyltransferases (PATs) that are defined by a conserved catalytic domain characterized by a DHHC sequence motif. Protein S-palmitoylation has a prominent role in regulating protein location, trafficking and function. Recent studies of DHHC PATs and their functional effects have demonstrated that their dysregulation is associated with human diseases, including schizophrenia, X-linked mental retardation, and Huntington's Disease. A growing number of reports indicate an important role for DHHC proteins and their substrates in tumorigenesis. Whereas DHHC PATs comprise a family of 23 enzymes in humans, a smaller number of enzymes that remove palmitate have been identified and characterized as potential therapeutic targets. Here we review current knowledge of the enzymes that mediate reversible palmitoylation and their cancer-associated substrates and discuss potential therapeutic applications.

Lipid rafts: integrated platforms for vascular organization offering therapeutic opportunities

Research on the nanoscale membrane structures known as lipid rafts is relevant to the fields of cancer biology, inflammation and ischaemia. Lipid rafts recruit molecules critical to signalling and regulation of the invasion process in malignant cells, the leukocytes that provide immunity in inflammation and the endothelial cells that build blood and lymphatic vessels, as well as the patterning of neural networks. As angiogenesis is a common denominator, regulation of receptors and signalling molecules critical to angiogenesis is central to the design of new approaches aimed at reducing, promoting or normalizing the angiogenic process. The goal of this review is to highlight some of the key issues that indicate the involvement of endothelial cell lipid rafts at each step of so-called 'sprouting angiogenesis', from stimulation of the vascular endothelial growth factor to the choice of tip cells, activation of migratory and invasion pathways, recruitment of molecules that guide axons in vascular patterning and maturation of blood vessels. Finally, the review addresses opportunities for future studies to define how these lipid domains (and their constituents) may be manipulated to stimulate the so-called 'normalization' of vascular networks within tumors, and be identified as the main target, enabling the development of more efficient chemotherapeutics and cancer immunotherapies.

The DAP-kinase interactome

DAP-kinase (DAPK) is a Ca(2+)/calmodulin regulated Ser/Thr kinase that activates a diverse range of cellular activities. It is subject to multiple layers of regulation involving both intramolecular signaling, and interactions with additional proteins, including other kinases and phosphatases. Its protein stability is modulated by at least three distinct ubiquitin-dependent systems. Like many kinases, DAPK participates in several signaling cascades, by phosphorylating additional kinases such as ZIP-kinase and protein kinase D (PKD), or Pin1, a phospho-directed peptidyl-prolyl isomerase that regulates the function of many phosphorylated proteins. Other substrate targets have more direct cellular effects; for example, phosphorylation of the myosin II regulatory chain and tropomyosin mediate some of DAPK's cytoskeletal functions, including membrane blebbing during cell death and cell motility. DAPK induces distinct death pathways of apoptosis, autophagy and programmed necrosis. Among the substrates implicated in these processes, phosphorylation of PKD, Beclin 1, and the NMDA receptor has been reported. Interestingly, not all cellular effects are mediated by DAPK's catalytic activity. For example, by virtue of protein-protein interactions alone, DAPK activates pyruvate kinase isoform M2, the microtubule affinity regulating kinases and inflammasome protein NLRP3, to promote glycolysis, influence microtubule dynamics, and enhance interleukin-1β production, respectively. In addition, a number of other substrates and interacting proteins have been identified, the physiological significance of which has not yet been established. All of these substrates, effectors and regulators together comprise the DAPK interactome. By presenting the components of the interactome network, this review will clarify both the mechanisms by which DAPK function is regulated, and by which it mediates its various cellular effects.

Overexpression of the dependence receptor UNC5H4 inhibits cell migration and invasion, and triggers apoptosis in neuroblastoma cell

UNC5H4 is a newly identified member of the UNC5H receptor family. Previously, we have demonstrated that UNC5H4 expression is significantly higher in favorable neuroblastomas than in unfavorable ones, and higher UNC5H4 level is correlated with longer survival time. However, the function of UNC5H4 in the tumorigenesis of neuroblastoma still remains elusive. In the present study, the effects of UNC5H4 overexpression on neuroblastoma SH-SY5Y cells were investigated. We showed that enforced expression of UNC5H4 receptor significantly inhibited anchorage-dependent and anchorage-independent growth of SH-SY5Y cells. Cell migration and invasion of SH-SY5Y cells transfected with UNC5H4-expressing plasmid were obviously suppressed as compared to those transfected with emptor vector or non-transfected cells. Moreover, overexpression of UNC5H4 resulted in apoptosis in SH-SY5Y cells. The induction of apoptosis by UNC5H4 was completely abolished in the presence of its ligand, netrin-1. Finally, caspase cleavage and the presence of death domain were required for UNC5H4 to induce apoptosis in neuroblastoma SH-SY5Y cells. These data suggest that the dependence receptor UNC5H4 may act as a putative tumor suppressor in neuroblastoma.

The dependence receptor UNC5H2/B triggers apoptosis via PP2A-mediated dephosphorylation of DAP kinase

The UNC5H dependence receptors promote apoptosis in the absence of their ligand, netrin-1, and this is important for neuronal and vascular development and for limitation of cancer progression. UNC5H2 (also called UNC5B) triggers cell death through the activation of the serine-threonine protein kinase DAPk. While performing a siRNA screen to identify genes implicated in UNC5H-induced apoptosis, we identified the structural subunit PR65β of the holoenzyme protein phosphatase 2A (PP2A). We show that UNC5H2/B recruits a protein complex that includes PR65β and DAPk and retains PP2A activity. PP2A activity is required for UNC5H2/B-induced apoptosis, since it activates DAPk by triggering its dephosphorylation. Moreover, netrin-1 binding to UNC5H2/B prevents this effect through interaction of the PP2A inhibitor CIP2A to UNC5H2/B. Thus we show here that, in the absence of netrin-1, recruitment of PP2A to UNC5H2/B allows the activation of DAPk via a PP2A-mediated dephosphorylation and that this mechanism is involved in angiogenesis regulation.

Dependence receptors: mechanisms of an announced death

Dependence receptors form a family of functionally related receptors which are all able to induce two completely opposite intracellular signals depending on the availability of their ligand. Indeed, in its presence, they mediate a positive, classical signal transduction of survival, differentiation or migration but without it, they trigger a negative signal which leads to cell death. The molecular mechanisms involved in triggering cell death in the absence of ligand are starting to be unravelled: dependence receptors are recruited at well-defined domains at the plasma membrane, they trigger cell death through a monomeric form, they are cleaved by caspases and they recruit a caspase activating complex.

Palmitoylation of membrane proteins (Review)

S-Palmitoylation is a reversible post-translational modification that results in the addition of a C16-carbon saturated fatty acyl chain to cytoplasmic cysteine residues. This modification is mediated by Palmitoyl-acyl Transferases that are starting to be investigated, and reversed by Protein Palmitoyl Thioesterases, which remain enigmatic. Palmitoylation of cytoplasmic proteins has been well described to regulate the interaction of these soluble proteins with specific membranes or membrane domains. Less is known about the consequences of palmitoylation in transmembrane proteins not only due to the dual difficulty of following a lipid modification and dealing with membrane proteins, but also due to the complexity of the palmitoylation-induced behavior. Moreover, possibly because the available data set is limited, the change in behavior induced by palmitoylation of a transmembrane protein is currently not predictable. We here review the various consequences reported for the palmitoylation of membrane proteins, which include improper folding in the endoplasmic reticulum, retention in the Golgi, inability to assemble into protein platforms, altered signaling capacity, premature endocytosis and missorting in the endocytic pathway. We then discuss the possible underlying mechanisms, in particular the ability of palmitoylation to control the conformation of transmembrane segments, to modify the affinity of a membrane protein for specific membrane domains and to control protein-protein interactions.