ECM deposition is driven by caveolin-1-dependent regulation of exosomal biogenesis and cargo sorting

The composition and physical properties of the extracellular matrix (ECM) critically influence tumor progression, but the molecular mechanisms underlying ECM layering are poorly understood. Tumor–stroma interaction critically depends on cell communication mediated by exosomes, small vesicles generated within multivesicular bodies (MVBs). We show that caveolin-1 (Cav1) centrally regulates exosome biogenesis and exosomal protein cargo sorting through the control of cholesterol content at the endosomal compartment/MVBs. Quantitative proteomics profiling revealed that Cav1 is required for exosomal sorting of ECM protein cargo subsets, including Tenascin-C (TnC), and for fibroblast-derived exosomes to efficiently deposit ECM and promote tumor invasion. Cav1-driven exosomal ECM deposition not only promotes local stromal remodeling but also the generation of distant ECM-enriched stromal niches in vivo. Cav1 acts as a cholesterol rheostat in MVBs, determining sorting of ECM components into specific exosome pools and thus ECM deposition. This supports a model by which Cav1 is a central regulatory hub for tumor–stroma interactions through a novel exosome-dependent ECM deposition mechanism.

The Calcineurin Variant CnAβ1 Controls Mouse Embryonic Stem Cell Differentiation by Directing mTORC2 Membrane Localization and Activation

Embryonic stem cells (ESC) have the potential to generate all the cell lineages that form the body. However, the molecular mechanisms underlying ESC differentiation and especially the role of alternative splicing in this process remain poorly understood. Here, we show that the alternative splicing regulator MBNL1 promotes generation of the atypical calcineurin Aβ variant CnAβ1 in mouse ESCs (mESC). CnAβ1 has a unique C-terminal domain that drives its localization mainly to the Golgi apparatus by interacting with Cog8. CnAβ1 regulates the intracellular localization and activation of the mTORC2 complex. CnAβ1 knockdown results in delocalization of mTORC2 from the membrane to the cytoplasm, inactivation of the AKT/GSK3β/β-catenin signaling pathway, and defective mesoderm specification. In summary, here we unveil the structural basis for the mechanism of action of CnAβ1 and its role in the differentiation of mESCs to the mesodermal lineage.

Rac1 nucleocytoplasmic shuttling drives nuclear shape changes and tumor invasion

Nuclear membrane microdomains are proposed to act as platforms for regulation of nuclear function, but little is known about the mechanisms controlling their formation. Organization of the plasma membrane is regulated by actin polymerization, and the existence of an actin pool in the nucleus suggests that a similar mechanism might operate here. We show that nuclear membrane organization and morphology are regulated by the nuclear level of active Rac1 through actin polymerization-dependent mechanisms. Rac1 nuclear export is mediated by two internal nuclear export signals and through its interaction with nucleophosmin-1 (B23), which acts as a Rac1 chaperone inside the nucleus. Rac1 nuclear accumulation alters the balance between cytosolic Rac1 and Rho, increasing RhoA signaling in the cytoplasm and promoting a highly invasive phenotype. Nuclear Rac1 shuttling is a finely tuned mechanism for controlling nuclear shape and organization and cell invasiveness.

Biomechanical remodeling of the microenvironment by stromal caveolin-1 favors tumor invasion and metastasis

Mechanotransduction is a key determinant of tissue homeostasis and tumor progression. It is driven by intercellular adhesions, cell contractility, and forces generated within the microenvironment and is dependent on extracellular matrix composition, organization, and compliance. We show that caveolin-1 (Cav1) favors cell elongation in three-dimensional cultures and promotes Rho- and force-dependent contraction, matrix alignment, and microenvironment stiffening through regulation of p190RhoGAP. In turn, microenvironment remodeling by Cav1 fibroblasts forces cell elongation. Cav1-deficient mice have disorganized stromal tissue architecture. Stroma associated with human carcinomas and melanoma metastases is enriched in Cav1-expressing carcinoma-associated fibroblasts (CAFs). Cav1 expression in breast CAFs correlates with low survival, and Cav1 depletion in CAFs decreases CAF contractility. Consistently, fibroblast expression of Cav1, through p190RhoGAP regulation, favors directional migration and invasiveness of carcinoma cells in vitro. In vivo, stromal Cav1 remodels peri- and intratumoral microenvironments to facilitate tumor invasion, correlating with increased metastatic potency. Thus, Cav1 modulates tissue responses through force-dependent architectural regulation of the microenvironment.

Coronin 1A promotes a cytoskeletal-based feedback loop that facilitates Rac1 translocation and activation

The activation of the Rac1 GTPase during cell signalling entails its translocation from the cytosol to membranes, release from sequestering Rho GDP dissociation inhibitors (RhoGDI), and GDP/GTP exchange. In addition to those steps, we show here that optimal Rac1 activation during cell signalling requires the engagement of a downstream, cytoskeletal-based feedback loop nucleated around the cytoskeletal protein coronin 1A and the Rac1 exchange factor ArhGEF7. These two proteins form a cytosolic complex that, upon Rac1-driven F-actin polymerization, translocates to juxtamembrane areas where it expands the pool of activated, membrane-bound Rac1. Such activity requires the formation of an F-actin/ArhGEF7-dependent physical complex of coronin 1A with Pak1 and RhoGDIα that, once assembled, promotes the Pak1-dependent dissociation of Rac1 from the Rac1/RhoGDIα complex and subsequent Rac1 activation. Genetic evidence demonstrates that this relay circuit is essential for generating sustained Rac1 activation levels during cell signalling.

Binding of CAP70 to inducible nitric oxide synthase and implications for the vectorial release of nitric oxide in polarized cells

In this article we analyze the mechanisms by which the C-terminal four amino acids of inducible nitric oxide synthase (NOS2) interact with proteins that contain PDZ (PSD-95/DLG/ZO-1) domains resulting in the translocation of NOS2 to the cellular apical domain. It has been reported that human hepatic NOS2 associates to EBP50, a protein with two PDZ domains present in epithelial cells. We describe herein that NOS2 binds through its four carboxy-terminal residues to CAP70, a protein that contains four PDZ modules that is targeted to apical membranes. Interestingly, this interaction augments both the cytochrome c reductase and .NO-synthase activities of NOS2. Binding of CAP70 to NOS2 also results in an increase in the population of active NOS2 dimers. In addition, CAP70 participates in the correct subcellular targeting of NOS2 in a process that is also dependent on the acylation state of the N-terminal end of NOS2. Hence, nonpalmitoylated NOS2 is unable to progress toward the apical side of the cell despite its interaction with either EBP50 or CAP70. Likewise, if we abrogate the interaction of NOS2 with either EBP50 or CAP70 by fusing the GFP reporter to the carboxy-terminal end of NOS2 palmitoylation is not sufficient to confer an apical targeting.

N-terminal palmitoylation within the appropriate amino acid environment conveys on NOS2 the ability to progress along the intracellular sorting pathways

We have analysed the mechanism by which palmitoylation permits the progression of nitric oxide synthase 2 (NOS2) along the ER-Golgi-TGN pathway. Introduction of an additional myristoylation site at the N-terminus of NOS2 resulted in a chimera that displayed an enhanced association with the particulate fraction and with the plasma membrane but did not display increased enzymatic activity. In the absence of palmitoylation, introduction of a surrogate myristoylation site resulted in a mutant NOS2 with only 25% activity compared with the wild-type enzyme. Hence, the novel surrogate myristoyl moiety not only failed to increase NOS2 activity when introduced in a wild-type sequence environment, but was also unable to rescue the inactive phenotype of the Cys3Ser mutant. Introduction of an additional palmitoylatable Cys at position 2 of the wild-type sequence resulted in a chimera that associated to a larger degree with membranes and displayed decreased activity. Our data indicate that palmitoylation of inducible NOS at position 3 exquisitely determines its transit along the secretory pathway following a route that cannot be mimicked by a surrogate myristoylation or by a palmitate at position 2. In addition, the exit of NOS2 from the TGN and the accumulation in the cellular plasma membrane per se did not correlate with increased ·NO synthesis.

Palmitoylation of Inducible Nitric-oxide Synthase at Cys-3 Is Required for Proper Intracellular Traffic and Nitric Oxide Synthesis

A number of cell types express inducible nitric-oxide synthase (NOS2) in response to exogenous insults such as bacterial lipopolysaccharide or proinflammatory cytokines. Although it has been known for some time that the N-terminal end of NOS2 suffers a post-translational modification, its exact identification has remained elusive. Using radioactive fatty acids, we show herein that NOS2 becomes thioacylated at Cys-3 with palmitic acid. Site-directed mutagenesis of this single residue results in the absence of the radiolabel incorporation. Acylation of NOS2 is completely indispensable for intracellular sorting and .NO synthesis. In fact, a C3S mutant of NOS2 is completely inactive and accumulates to intracellular membranes that almost totally co-localize with the Golgi marker beta-cop. Likewise, low concentrations of the palmitoylation blocking agents 2-Br-palmitate or 8-Br-palmitate severely affected the .NO synthesis of both NOS2 induced in muscular myotubes and transfected NOS2. However, unlike endothelial NOS, palmitoylation of inducible NOS is not involved in its targeting to caveolae. We have created 16 NOS2-GFP chimeras to inspect the effect of the neighboring residues of Cys-3 on the degree of palmitoylation. In this regard, the hydrophobic residue Pro-4 and the basic residue Lys-6 seem to be indispensable for palmitoylation. In addition, agents that block the endoplasmic reticulum to Golgi transit such as brefeldin A and monensin drastically reduced NOS2 activity leading to its accumulation in perinuclear areas. In summary, palmitoylation of NOS2 at Cys-3 is required for both its activity and proper intracellular localization.

Proteomic identification of brain proteins that interact with dynein light chain LC8

Cytoplasmic dynein is a large minus end-directed microtubule motor that translocates cargos towards the minus end of microtubules. Light chain 8 of the dynein machinery (LC8) has been reported to interact with a large variety of proteins that possess K/RSTQT or GIQVD motifs in their sequence, hence permitting their transport in a retrograde manner. Yeast two-hybrid analysis has revealed that in brain, LC8 associates directly with several proteins such as neuronal nitric oxide synthase, guanylate kinase domain-associated protein and gephyrin. In this work, we report the identification of over 40 polypeptides, by means of a proteomic approach, that interact with LC8 either directly or indirectly. Many of the neuronal proteins that we identified cluster at the post-synaptic terminal, and some of them such as phosphofructokinase, lactate dehydrogenase or aldolase are directly involved in glutamate metabolism. Other pool of proteins identified displayed the LC8 consensus binding motif. Finally, recombinant LC8 was produced and a library of overlapping dodecapeptides (pepscan) was employed to map the LC8 binding site of some of the proteins that were previously identified using the proteomic approach, hence confirming binding to the consensus binding sites.

Induction of nitric oxide synthase-2 proceeds with the concomitant downregulation of the endogenous caveolin levels

Several cell types express inducible nitric oxide synthase (NOS2) in response to exogenous insults such as bacterial lipopolysaccharide (LPS) or proinflammatory cytokines. For instance, muscular cells treated with LPS and interferon gamma (IFN-gamma) respond by increasing the mRNA and protein levels of NOS2, and synthesize large amounts of nitric oxide. We show here that transcriptional induction of NOS2 in muscular cells proceeds with a concomitant decrease in the levels of caveolin-1, -2 and -3. Addition of *NO-releasing compounds to C2C12 muscle cells reveals that this downregulation of the caveolin (cav) levels is due to the presence of *NO itself in the case of caveolin-3 and to the action of the LPS/IFN-gamma in the case of cav-1 and cav-2. Likewise, muscle cells obtained from NOS2(-/-) knockout mice challenged with LPS/IFN-gamma could downregulate their levels of cav-1 but not of cav-3, unlike wild-type animals, in which both cav-1 and cav-3 levels diminished in the presence of the proinflammatory insult. Laser confocal immunofluorescence analysis proves that *NO exerts autocrine and paracrine actions, hence diminishing the cav-3 levels. When the induced NOS2 was purified using an affinity resin or immunoprecipitated from muscular tissues, it appears strongly bound not only to calmodulin but also to cav-1, and marginally to cav-2 and cav-3. When the cav levels where reduced using antisense oligonucleotides, an increase in the NOS2-derived.NO levels could be measured, demonstrating the inhibitory role of the three cav isoforms. Our results show that cells expressing NOS2 diminish their cav levels when the synthesis of *NO is required.

Recognition of novel viral sequences that associate with the dynein light chain LC8 identified through a pepscan technique

Recent data from multiple laboratories indicate that upon infection, many different families of viruses hijack the dynein motor machinery and become transported in a retrograde manner towards the cell nucleus. In certain cases, one of the dynein light chains, LC8, is involved in this interaction. Using a library of overlapping dodecapeptides synthesized on a cellulose membrane (pepscan technique) we have analyzed the interaction of the dynein light chain LC8 with 17 polypeptides of viral origin. We demonstrate the strong binding of two herpesvirus polypeptides, the human adenovirus protease, vaccinia virus polymerase, human papillomavirus E4 protein, yam mosaic virus polyprotein, human respiratory syncytial virus attachment glycoprotein, human coxsackievirus capsid protein and the product of the AMV179 gene of an insect poxvirus to LC8. Our data corroborate the manipulation of the dynein macromolecular complex of the cell during viral infection and point towards the light chain LC8 as one of the most frequently used targets of virus manipulation.

Distance-dependent cellular palmitoylation of de-novo-designed sequences and their translocation to plasma membrane subdomains

Using recursive PCR, we created an artificial protein sequence that consists of a consensus myristoylation motif (MGCTLS) followed by the triplet AGS repeated nine times and fused to the GFP reporter. This linker-GFP sequence was utilized as a base to produce multiple mutants that were used to transfect COS-7 cells. Constructs where a `palmitoylable' cysteine residue was progressively moved apart from the myristoylation site to positions 3, 9, 15 and 21 of the protein sequence were made, and these mutants were used to investigate the effect of protein myristoylation on subsequent palmitoylation,subcellular localization, membrane association and caveolin-1 colocalization. In all cases, dual acylation of the GFP chimeras correlated with translocation to Triton X-100-insoluble cholesterol/sphingomyelin-enriched subdomains. Whereas a strong Golgi labeling was observed in all the myristoylated chimeras, association with the plasma membrane was only observed in the dually acylated constructs. Taking into account the conflicting data regarding the existence and specificity of cellular palmitoyl-transferases, our results provide evidence that de-novo-designed sequences can be efficiently S-acylated with palmitic acid in vivo, strongly supporting the hypothesis that non-enzymatic protein palmitoylation can occur within mammalian cells. Additionally, this palmitoylation results in the translocation of the recombinant construct to low-fluidity domains in a myristate-palmitate distance-dependent manner.