SUN2 regulates mitotic duration in response to extracellular matrix rigidity.

How cells adjust their growth to the spatial and mechanical constraints of their surrounding environment is central to many aspects of biology. Here, we examined how extracellular matrix (ECM) rigidity affects cell division. We found that cells divide more rapidly when cultured on rigid substrates. While we observed no effect of ECM rigidity on rounding or postmitotic spreading duration, we found that changes in matrix stiffness impact mitosis progression. We noticed that ECM elasticity up-regulates the expression of the linker of nucleoskeleton and cytoskeleton (LINC) complex component SUN2, which in turn promotes metaphase-to-anaphase transition by acting on mitotic spindle formation, whereas when cells adhere to soft ECM, low levels of SUN2 expression perturb astral microtubule organization and delay the onset of anaphase.

Seipin localizes at endoplasmic-reticulum-mitochondria contact sites to control mitochondrial calcium import and metabolism in adipocytes.

Deficiency of the endoplasmic reticulum (ER) protein seipin results in generalized lipodystrophy by incompletely understood mechanisms. Here, we report mitochondrial abnormalities in seipin-deficient patient cells. A subset of seipin is enriched at ER-mitochondria contact sites (MAMs) in human and mouse cells and localizes in the vicinity of calcium regulators SERCA2, IP3R, and VDAC. Seipin association with MAM calcium regulators is stimulated by fasting-like stimuli, while seipin association with lipid droplets is promoted by lipid loading. Acute seipin removal does not alter ER calcium stores but leads to defective mitochondrial calcium import accompanied by a widespread reduction in Krebs cycle metabolites and ATP levels. In mice, inducible seipin deletion leads to mitochondrial dysfunctions preceding the development of metabolic complications. Together, these data suggest that seipin controls mitochondrial energy metabolism by regulating mitochondrial calcium influx at MAMs. In seipin-deficient adipose tissue, reduced ATP production compromises adipocyte properties, contributing to lipodystrophy pathogenesis.

PCSK9 is not secreted from mature differentiated intestinal cells.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) promotes lysosomal degradation of the LDL receptor and is a key regulator of cholesterol metabolism. After the liver, the small intestine is the second organ that highly expresses PCSK9. However, the small intestine's ability to secrete PCSK9 remains a matter of debate. While liver-specific PCSK9-deficient mice present no PCSK9 in systemic blood, human intestinal Caco-2 cells can actively secrete PCSK9. This raises the possibility for active intestinal secretion via the portal blood. Here, we aimed to determine whether enterocytes can secrete PCSK9 using in vitro, ex vivo, and in vivo approaches. We first observed that PCSK9 secretion from Caco-2 cells was biphasic and dependent on Caco-2 maturation status. Transcriptional analysis suggested that this transient reduction in PCSK9 secretion might be due to loss of SREBP2-mediated transcription of PCSK9. Consistently, PCSK9 secretion was not detected ex vivo in human or mouse intestinal biopsies mounted in Ussing chambers. Finally, direct comparison of systemic versus portal blood PCSK9 concentrations in WT or liver-specific PCSK9-deficient mice confirmed the inability of the small intestine to secrete PCSK9 into the portal compartment. Altogether, our data demonstrate that mature enterocytes do not secrete PCSK9 and reinforce the central role of the liver in the regulation of the concentration of circulating PCSK9 and consequently of cellular LDL receptors.

Hepatitis B virus Core protein nuclear interactome identifies SRSF10 as a host RNA-binding protein restricting HBV RNA production.

Despite the existence of a preventive vaccine, chronic infection with Hepatitis B virus (HBV) affects more than 250 million people and represents a major global cause of hepatocellular carcinoma (HCC) worldwide. Current clinical treatments, in most of cases, do not eliminate viral genome that persists as a DNA episome in the nucleus of hepatocytes and constitutes a stable template for the continuous expression of viral genes. Several studies suggest that, among viral factors, the HBV core protein (HBc), well-known for its structural role in the cytoplasm, could have critical regulatory functions in the nucleus of infected hepatocytes. To elucidate these functions, we performed a proteomic analysis of HBc-interacting host-factors in the nucleus of differentiated HepaRG, a surrogate model of human hepatocytes. The HBc interactome was found to consist primarily of RNA-binding proteins (RBPs), which are involved in various aspects of mRNA metabolism. Among them, we focused our studies on SRSF10, a RBP that was previously shown to regulate alternative splicing (AS) in a phosphorylation-dependent manner and to control stress and DNA damage responses, as well as viral replication. Functional studies combining SRSF10 knockdown and a pharmacological inhibitor of SRSF10 phosphorylation (1C8) showed that SRSF10 behaves as a restriction factor that regulates HBV RNAs levels and that its dephosphorylated form is likely responsible for the anti-viral effect. Surprisingly, neither SRSF10 knock-down nor 1C8 treatment modified the splicing of HBV RNAs but rather modulated the level of nascent HBV RNA. Altogether, our work suggests that in the nucleus of infected cells HBc interacts with multiple RBPs that regulate viral RNA metabolism. Our identification of SRSF10 as a new anti-HBV restriction factor offers new perspectives for the development of new host-targeted antiviral strategies.

Leukocyte RhoA exchange factor Arhgef1 mediates vascular inflammation and atherosclerosis.

Abnormal activity of the renin-angiotensin-aldosterone system plays a causal role in the development of hypertension, atherosclerosis, and associated cardiovascular events such as myocardial infarction, stroke, and heart failure. As both a vasoconstrictor and a proinflammatory mediator, angiotensin II (Ang II) is considered a potential link between hypertension and atherosclerosis. However, a role for Ang II-induced inflammation in atherosclerosis has not been clearly established, and the molecular mechanisms and intracellular signaling pathways involved are not known. Here, we demonstrated that the RhoA GEF Arhgef1 is essential for Ang II-induced inflammation. Specifically, we showed that deletion of Arhgef1 in a murine model prevents Ang II-induced integrin activation in leukocytes, thereby preventing Ang II-induced recruitment of leukocytes to the endothelium. Mice lacking both LDL receptor (LDLR) and Arhgef1 were protected from high-fat diet-induced atherosclerosis. Moreover, reconstitution of Ldlr-/- mice with Arhgef1-deficient BM prevented high-fat diet-induced atherosclerosis, while reconstitution of Ldlr-/- Arhgef1-/- with WT BM exacerbated atherosclerotic lesion formation, supporting Arhgef1 activation in leukocytes as causal in the development of atherosclerosis. Thus, our data highlight the importance of Arhgef1 in cardiovascular disease and suggest targeting Arhgef1 as a potential therapeutic strategy against atherosclerosis.

Angiotensin II activates the RhoA exchange factor Arhgef1 in humans.

Although a causative role for RhoA-Rho kinase has been recognized in the development of human hypertension, the molecular mechanism(s) and the RhoA guanine exchange factor(s) responsible for the overactivation of RhoA remain unknown. Arhgef1 was identified as a RhoA guanine exchange factor involved in angiotensin II (Ang II)-mediated regulation of vascular tone and hypertension in mice. The aim of this study was to determine whether Arhgef1 is activated and involved in the activation of RhoA-Rho kinase signaling by Ang II in humans. In vitro stimulation of human coronary artery smooth muscle cells and human peripheral blood mononuclear cells by Ang II (0.1 µmol/L) induced activation of Arhgef1 attested by its increased tyrosine phosphorylation. Silencing of Arhgef1 expression by siRNA inhibited Ang II-induced activation of RhoA-Rho kinase signaling. In normotensive subjects, activation of the renin-angiotensin system by a low-salt diet for 7 days increased RhoA-Rho kinase signaling and stimulated Arhgef1 activity in peripheral blood mononuclear cells. In conclusion, our results strongly suggest that Arhgef1 mediates Ang II-induced RhoA activation in humans. Moreover, they show that measurement of RhoA guanine exchange factor activity in peripheral blood mononuclear cells might be a useful method to evaluate RhoA guanine exchange factor activity in humans.

RhoA guanine exchange factor expression profile in arteries: evidence for a Rho kinase-dependent negative feedback in angiotensin II-dependent hypertension.

Sustained overactivation of RhoA is a common component for the pathogenesis of several cardiovascular disorders, including hypertension. Although activity of Rho proteins depends on Rho exchange factors (Rho-GEFs), the identity of Rho-GEFs expressed in vascular smooth muscle cells (VSMC) and participating in the control of Rho protein activity and Rho-dependent functions remains unknown. To address this question, we analyzed by quantitative RT-PCR the expression profile of 28 RhoA-GEFs in arteries of normotensive (saline-treated) and hypertensive (ANG II-treated) rats. Sixteen RhoA-GEFs were downregulated in mesenteric arteries of hypertensive rats, among which nine are also downregulated in cultured VSMC stimulated by ANG II (100 nM, 48 h), suggesting a direct effect of ANG II. Inhibition of type 1 ANG II receptors (losartan, 1 µM) or Rho kinase (fasudil, 10 µM) prevented ANG II-induced RhoA-GEF downregulation. Functionally, ANG II-induced downregulation of RhoA-GEFs is associated with decreased Rho kinase activation in response to endothelin-1, norepinephrine, and U-46619. This work thus identifies a group of RhoA-GEFs that controls RhoA and RhoA-dependent functions in VSMC, and a negative feedback of RhoA/Rho kinase activity on the expression of these RhoA-GEFs that may play an adaptative role to limit RhoA/Rho kinase activation.

Stable producer cell lines for adeno-associated virus (AAV) assembly.

Stable producer cell lines containing both the rep and cap genes and recombinant adeno-associated virus (rAAV) vectors can be infected with a helper virus to provide reliable and efficient production of rAAV stocks. However, the development of these cell lines is time-consuming. The procedure described here is therefore recommended only for studies requiring the production of high amounts of rAAV, such as preclinical studies performed in large animals.

Relative influence of the adeno-associated virus (AAV) type 2 p5 element for recombinant AAV vector site-specific integration.

The p5 promoter region of the adeno-associated virus type 2 (AAV-2) rep gene has been described as essential for Rep-mediated site-specific integration (RMSSI) of plasmid sequences in human chromosome 19. We report here that insertion of a full-length or minimal p5 element between the viral inverted terminal repeats does not significantly increase RMSSI of a recombinant AAV (rAAV) vector after infection of growth-arrested or proliferating human cells. This result suggests that the p5 element may not improve RMSSI of rAAV vectors in vivo.

Impact of the interaction between herpes simplex virus type 1 regulatory protein ICP0 and ubiquitin-specific protease USP7 on activation of adeno-associated virus type 2 rep gene expression.

Expression of the herpes simplex virus type 1 (HSV-1) regulatory protein ICP0 in transfected cells reactivates rep gene expression from integrated adeno-associated virus (AAV) type 2 genomes via a mechanism that requires both its RING finger and USP7 interaction domains. In this study, we found that the rep reactivation defect of USP7-binding-negative ICP0 mutants can be overcome by further deletion of sequences in the C-terminal domain of ICP0, indicating that binding of USP7 to ICP0 is not directly required. Unlike the case in transfected cells, only the RING finger domain of ICP0 was essential for rep gene reactivation during HSV-1 infection. However, mutants unable to bind to USP7 activate HSV-1 gene expression and reactivate rep gene expression with reduced efficiencies. These results further elucidate the role of ICP0 as a helper factor for AAV replication and illustrate that care is required when extrapolating from the properties of ICP0 in transfection assays to events occurring during HSV-1 infection.

The cellular TATA binding protein is required for rep-dependent replication of a minimal adeno-associated virus type 2 p5 element.

The p5 promoter region of adeno-associated virus type 2 (AAV-2) is a multifunctional element involved in rep gene expression, Rep-dependent replication, and site-specific integration. We initially characterized a 350-bp p5 region by its ability to behave like a cis-acting replication element in the presence of Rep proteins and adenoviral factors. The objective of this study was to define the minimal elements within the p5 region required for Rep-dependent replication. Assays performed in transfected cells (in vivo) indicated that the minimal p5 element was composed by a 55-bp sequence (nucleotides 250 to 304 of wild-type AAV-2) containing the TATA box, the Rep binding site, the terminal resolution site present at the transcription initiation site (trs(+1)), and a downstream 17-bp region that could potentially form a hairpin structure localizing the trs(+1) at the top of the loop. Interestingly, the TATA box was absolutely required for in vivo but dispensable for in vitro, i.e., cell-free, replication. We also demonstrated that Rep binding and nicking at the trs(+1) was enhanced in the presence of the cellular TATA binding protein, and that overexpression of this cellular factor increased in vivo replication of the minimal p5 element. Together, these studies identified the minimal replication origin present within the AAV-2 p5 promoter region and demonstrated for the first time the involvement of the TATA box, in cis, and of the TATA binding protein, in trans, for Rep-dependent replication of this viral element.

Evidence for encapsidation of prokaryotic sequences during recombinant adeno-associated virus production and their in vivo persistence after vector delivery.

Recombinant adeno-associated virus vectors (rAAV) have been successfully used for long-term gene expression in animal models and in patients. However, while the therapeutic potential of rAAV appears promising, safety issues, including contaminants found in vector stocks, must be further evaluated. We previously reported that a cis-acting replication element present within the AAV-2 p5 promoter was responsible for the encapsidation of rep-cap sequences observed during rAAV production. In that study, we also noticed that plasmid-derived prokaryotic sequences (such as the ampicillin resistance gene) could be found packaged into AAV capsids. In this report, first we confirmed and extended the latter observation by analyzing rAAV stocks produced using different procedures. Second, we demonstrated that these plasmid-derived sequences were transferred and persisted in vivo after rAAV injection into different tissues. Third, our data showed that at least some of these packaged plasmid molecules were linked to the AAV ITRs and were present in vivo in a form that could be rescued through bacterial transformation. This study highlights the need for more stringent characterization of rAAV stocks and provides useful information on the development of rAAV production methods that are able to circumvent or limit the generation of such undesirable particles.

Evidence for packaging of rep-cap sequences into adeno-associated virus (AAV) type 2 capsids in the absence of inverted terminal repeats: a model for generation of rep-positive AAV particles.

We previously reported that a 350-bp region of the adeno-associated virus (AAV) type 2 rep gene contains a cis-acting element responsible for the Rep-dependent replication of a transiently transfected rep-cap plasmid. In this study, we further report that replicated rep-cap sequences can be packaged into AAV capsids in the absence of the inverted terminal repeats.