Xiaohui Zha

Atherosclerosis is a chronic inflammatory disease primarily initiated by the sub-endothelial retention and modification of low-density lipoproteins (LDL), such as oxidation and aggregation. While oxidized LDL (oxLDL) exists in atherosclerotic lesions, aggregated lipoproteins are also prominent. Aggregated LDL (agLDL), for instance, is viewed as a passive substrate for macrophages to uptake and form foam cells. Recent studies show that agLDL, through toll-like receptor 4 (TLR4), induces formation of lysosomal synapses, which drive agLDL degradation by macrophages, a process described as “digestive exophagy”. As TLR4 is one of the pattern-recognition receptors for inflammatory responses in macrophages, we postulated that agLDL could initiate macrophage inflammation along with lipoprotein degradation/internalization, thereby contributing to a low degree of chronic inflammation in atherosclerosis. Here, we demonstrate that agLDL directly activates the canonical NF-κB signaling pathway and triggers a pro-inflammatory transcriptional program through TLR4, which closely resembles classical lipopolysaccharide (LPS)-mediated TLR4 activation. In addition, agLDL also activates TLR4 to generate inflammatory macrophages that recruit immune cells, a key process in the development of atherosclerotic lesions. Our findings therefore support a novel dual functionality of agLDL: a lipid donor and an inflammatory agonist. This establishes the agLDL-TLR4-NF-κB axis as a direct link between LDL aggregation in the intima and the initiation of sterile vascular inflammation. It also strengthens the understanding of the role of digestive exophagy with new mechanistic details.

Fluorescence microscopy is a widely used tool for visualizing membrane organization in model and cellular systems; however, photoexcitation during imaging can generate reactive oxygen species, thereby altering membrane structure. Here, we show that fluorescence imaging actively drives membrane phase separation through photo-induced phospholipid oxidation. Using giant unilamellar vesicles, we observed that illumination of initially homogeneous membranes triggers the emergence of coexisting liquid-ordered and liquid-disordered domains. Chemical analysis by thin layer chromatography and mass spectrometry revealed the formation of a complex mixture of oxidized lipid species following excitation, characterized by stepwise oxygen additions to unsaturated phospholipids and multiple degradation products. These modifications increase lipid polarity and disrupt acyl chain packing, reducing favorable interactions with saturated phospholipids. As a result, membranes near the miscibility boundary become prone to demixing upon illumination, indicating that photo-induced lipid oxidation effectively shifts the miscibility boundary. Consistent with this model, light-induced phase separation is highly sensitive to membrane composition, with maximal effects observed near the liquid-liquid miscibility boundary. Electroformation substrate further modulated this effect, suggesting that pre-existing oxidative conditions sensitize membranes to subsequent photo-induced changes. Together, these findings demonstrate that excitation light can introduce artifacts in membrane phase behaviour. More broadly, subtle chemical modifications of phospholipids can shift membrane miscibility and reorganize membrane structure.

Statins are known to be anti-inflammatory, but the mechanism remains poorly understood. Here, we show that macrophages, either treated with statin in vitro or from statin-treated mice, have reduced cholesterol levels and higher expression of Jmjd3, a H3K27me3 demethylase. We provide evidence that lowering cholesterol levels in macrophages suppresses the adenosine triphosphate (ATP) synthase in the inner mitochondrial membrane and changes the proton gradient in the mitochondria. This activates nuclear factor kappa-B (NF-κB) and Jmjd3 expression, which removes the repressive marker H3K27me3. Accordingly, the epigenome is altered by the cholesterol reduction. When subsequently challenged by the inflammatory stimulus lipopolysaccharide (M1), macrophages, either treated with statins in vitro or isolated from statin-fed mice, express lower levels proinflammatory cytokines than controls, while augmenting anti-inflammatory Il10 expression. On the other hand, when macrophages are alternatively activated by IL-4 (M2), statins promote the expression of Arg1, Ym1, and Mrc1. The enhanced expression is correlated with the statin-induced removal of H3K27me3 from these genes prior to activation. In addition, Jmjd3 and its demethylase activity are necessary for cholesterol to modulate both M1 and M2 activation. We conclude that upregulation of Jmjd3 is a key event for the anti-inflammatory function of statins on macrophages.

Background and Objectives There is significant heterogeneity in the reporting of outcome measures in aneurysmal subarachnoid hemorrhage (aSAH) research. The modified Rankin scale (mRS) is the most commonly reported functional outcome measure. The mRS focuses on physical disability; however, many aSAH survivors experience sequalae in other domains, and the mRS may therefore not capture outcomes important to aSAH survivors. The objective of this study was to assess the clinical relevance of the mRS as a research outcome measure to people with lived aSAH experience. Methods We conducted an international cross-sectional survey of 355 aSAH survivors, family members, and caregivers to evaluate patient-perceived outcomes in relation to the mRS. The mRS was assessed using a previously validated web-based tool. Results Response rate was 60%; respondents from 7 continents were composed of 86% aSAH survivors and 14% family members/caregivers. Agreement between self-assessed outcome and the mRS was poor (Kappa 0.26 [CI 0.14–0.39]). Of the 172 respondents who self-assessed as having had a good aSAH outcome, 122 (71%) had a score of 0–2 on the mRS. Approximately 19% of respondents with a good outcome, based on a measured mRS score of 0–2, self-assessed as having had a poor aSAH outcome. When the mRS score was dichotomized as 0–3 corresponding to a good outcome, agreement between the score and self-assessed outcome remained poor with a Kappa score of 0.40 (CI 0.20–0.60). Approximately 30% of respondents believed that the mRS should not be used as an outcome measure in future aSAH trials. Discussion The findings suggest that there is poor agreement between aSAH survivors9 self-assessed outcome, their actual mRS score, and the dichotomization of the mRS score into good/poor outcomes. Patient-centered and patient-informed outcome measurement tools are needed to guide the aSAH research agenda.

Stains are known to be anti-inflammatory, but the mechanism remains poorly understood. Here we show that macrophages, either treated with statin in vitro or from statin-treated mice, have reduced cholesterol levels and higher expression of Jmjd3, a H3K27me3 demethylase. We provide evidence that lowering cholesterol levels in macrophages suppresses the ATP synthase in the inner mitochondrial membrane (IMM) and changes the proton gradient in the mitochondria. This activates NFκB and Jmjd3 expression to remove the repressive marker H3K27me3. Accordingly, the epigenome is altered by the cholesterol reduction. When subsequently challenged by the inflammatory stimulus LPS (M1), both macrophages treated with statins in vitro or isolated from statin-treated mice in vivo , express lower levels pro-inflammatory cytokines than controls, while augmenting anti-inflammatory Il10 expression. On the other hand, when macrophages are alternatively activated by IL4 (M2), statins promote the expression of Arg1 , Ym1 , and Mrc1 . The enhanced expression is correlated with the statin-induced removal of H3K27me3 from these genes prior to activation. In addition, Jmjd3 and its demethylase activity are necessary for cholesterol to modulate both M1 and M2 activation. We conclude that upregulation of Jmjd3 is a key event for the anti-inflammatory function of statins on macrophages.

Cholesterol plays an important role in macrophage functions including their immune response ¹ . Recently, NF- k B was shown to reprogram the epigenome in macrophages ² . Here, we show that NF-kB pathway is activated in resting macrophages when cholesterol is reduced by statin or methyl-β-cyclodextrin (MCD). Activated NF- k B increases the expression of histone-modifying enzymes, such as demethylase JMJD3. We provide evidence that the epigenome in these macrophages is reprogrammed, likely driven by NF- k B and histone modifications ² . We also show that cholesterol reduction in macrophages results in suppression of mitochondria respiration. Specifically, cholesterol levels in the inner membrane of the mitochondria is reduced, which impairs the efficiency of ATP synthase (complex V). Consequently, protons accumulate in the intermembrane space to active NF- k B and JMJD3, thereby modifying the epigenome. When subsequently challenged by the inflammatory stimulus lipopolysaccharide (LPS), cholesterol-reduced macrophages generate responses that are less pro-inflammatory and more homeostatic, which should favour inflammation resolution. Taken together, we describe a mechanism by which the level of mitochondrial cholesterol in resting macrophages regulates the epigenome through NF-kB, thereby preparing macrophage for future immune activation.

The plasma membrane in mammalian cells is rich in cholesterol, but how the cholesterol is partitioned between the two leaflets of the plasma membrane remains a matter of debate. Recently, Liu et al. used domain 4 (D4) of perfringolysin O as a cholesterol sensor to argue that cholesterol is mostly in the exofacial leaflet (Liu et al., 2017). This conclusion was made by interpreting D4 binding in live cells using in vitro calibrations with liposomes. However, liposomes may be unfaithful in mimicking the plasma membrane, as we demonstrate here. Also, D4 binding is highly sensitive to the presence of cytosolic proteins. In addition, we find that a D4 variant, which requires >35 mol% cholesterol to bind to liposomes in vitro, does in fact bind to the cytoplasmic leaflet of the plasma membrane in a cholesterol-dependent manner. Thus, we believe, based on the current evidence, that it is unlikely that there is a significantly higher proportion of cholesterol in the exofacial leaflet of the plasma membrane compared to the cytosolic leaflet.

Mammalian cell sphingolipids, primarily with C24 and C16 acyl chains, reside in the outer leaflet of the plasma membrane. Curiously, little is known how C24 sphingolipids impact cholesterol and membrane microdomains. Here, we generated giant unilamellar vesicles and live mammalian cells with C24 or C16 sphingomyelin exclusively in the outer leaflet and compared microdomain formation. In giant unilamellar vesicles, we observed that asymmetrically placed C24 sphingomyelin suppresses microdomains. Conversely, C16 sphingomyelin facilitates microdomains. Replacing endogenous sphingolipids with C24 or C16 sphingomyelin in live HeLa cells has a similar impact on microdomains, characterized by FRET between GPI-anchored proteins: C24, but not C16, sphingomyelin suppresses submicron domains in the plasma membrane. Molecular dynamics simulations indicated that, when in the outer leaflet, the acyl chain of C24 sphingomyelin interdigitates into the opposing leaflet, thereby favouring cholesterol in the inner leaflet. We indeed found that cholesterol prefers the inner over the outer leaflet of asymmetric unilamellar vesicles (80/20) when C24 sphingomyelin is in the outer leaflet. However, when C16 sphingomyelin is in the outer leaflet, cholesterol is evenly partitioned between leaflets (50/50). Interestingly, when a mixture of C24/C16 sphingomyelin is in the outer leaflet of unilamellar vesicles, cholesterol still prefers the inner leaflet (80/20). Indeed, in human erythrocyte plasma membrane, where a mixture of C24 and C16 sphingolipids are naturally in the outer leaflet, cholesterol prefers the cytoplasmic leaflet (80/20). Therefore, C24 sphingomyelin uniquely interacts with cholesterol and governs the lateral organization in asymmetric membranes, including the plasma membrane, potentially by generating cholesterol asymmetry. Statement of Significance: The plasma membrane bilayer of mammalian cells has distinct phospholipids between the outer and inner leaflet, with sphingolipids exclusively in the outer leaflet. A large portion of mammalian sphingolipids have very long acyl chains (C24). Little is known how C24 sphingolipids function in the outer leaflet. Mutations in the ceramide synthase 2 gene is found to decrease C24. This severely perturbs homeostasis in mice and humans. Here, we investigated unilamellar vesicles and mammalian cells with C24 sphingomyelin exclusively in the outer leaflet. We provide evidence that outer leaflet C24 sphingomyelin suppresses microdomains in model membranes and live cells by partitioning cholesterol into the inner leaflet. We propose that C24 sphingolipids are critical to the function of the plasma membrane.

MTORC1 is known to activate sterol regulatory element-binding proteins (SREBPs) including SREBP-2, a master regulator of cholesterol synthesis. Through incompletely understood mechanisms, activated mTORC1 triggers translocation of SREBP-2, an endoplasmic reticulum (ER) resident protein, to the Golgi where SREBP-2 is cleaved to translocate to the nucleus and activate gene expression for cholesterol synthesis. Low ER cholesterol is a well-established trigger for SREBP-2 activation. We thus investigated whether mTORC1 activates SREBP-2 by reducing cholesterol delivery to the ER. We report here that mTORC1 activation is accompanied by low ER cholesterol and an increase of SREBP-2 activation. Conversely, a decrease in mTORC1 activity coincides with a rise in ER cholesterol and a decrease in SERBP-2 activity. This rise in ER cholesterol is of lysosomal origin: blocking the exit of cholesterol from lysosomes by U18666A or NPC1 siRNA prevents ER cholesterol from increasing and, consequently, SREBP-2 is activated without mTORC1 activation. Furthermore, when mTORC1 activity is low, cholesterol is delivered to lysosomes through two membrane trafficking pathways: autophagy and rerouting of endosomes to lysosomes. Indeed, with dual blockade of both pathways by Atg5^−/− and dominant-negative rab5, ER cholesterol fails to increase when mTORC1 activity is low, and SREBP-2 is activated. Conversely, overexpressing constitutively active Atg7, which forces autophagy and raises ER cholesterol even when mTORC1 activity is high, suppresses SREBP-2 activation. We conclude that mTORC1 actively suppresses autophagy and maintains endosomal recycling, thereby preventing endosomes and autophagosomes from reaching lysosomes. This results in a reduction of cholesterol in the ER and activation of SREBP-2.