Distinct spatiotemporal contribution of morphogenetic events and mechanical tissue coupling during Xenopus neural tube closure.

Neural tube closure (NTC) is a fundamental process during vertebrate development and is indispensable for the formation of the central nervous system. Here, using Xenopus laevis embryos, live imaging, single-cell tracking, optogenetics and loss-of-function experiments, we examine the roles of convergent extension and apical constriction, and define the role of the surface ectoderm during NTC. We show that NTC is a two-stage process with distinct spatiotemporal contributions of convergent extension and apical constriction at each stage. Convergent extension takes place during the first stage and is spatially restricted at the posterior tissue, whereas apical constriction occurs during the second stage throughout the neural plate. We also show that the surface ectoderm is mechanically coupled with the neural plate and its movement during NTC is driven by neural plate morphogenesis. Finally, we show that an increase in surface ectoderm resistive forces is detrimental for neural plate morphogenesis.

FAK displacement from focal adhesions: a promising strategy to target processes implicated in cancer progression and metastasis.

BACKGROUND: Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that is overexpressed or activated in several advanced-stage solid cancers. It is known to play both kinase-dependent and -independent roles in promoting tumor progression and metastasis. Numerous inhibitors, targeting either the enzymatic or scaffolding activities of FAK have been generated, with varying degree of success. Here, we describe a novel approach to site-specifically target both kinase-dependent and -independent FAK functions at focal adhesions (FAs), the primary sites at which the kinase exerts its activity. METHODS: We took advantage of the well-characterized interactions between the paxillin LD motifs and the FAK FAT domain and generated a polypeptide (LD2-LD3-LD4) expected to compete with interactions with paxillin. Co-immunoprecipitation experiments were performed to examine the interaction between the LD2-LD3-LD4 polypeptide and FAK. The effects of LD2-LD3-LD4 in the localization and functions of FAK, as well as FA composition, were evaluated using quantitative immunofluorescence, cell fractionation, FA isolation and Western Blot analysis. Live cell imaging, as well as 2-D migration and cell invasion assays were used to examine the effects on FA turnover and tumor cell migration and invasion. RESULTS: Expression of the LD2-LD3-LD4 polypeptide prevents FAK localization at FAs, in a controlled and dose-dependent manner, by competing with endogenous paxillin for FAK binding. Importantly, the LD2-LD3-LD4 peptide did not otherwise affect FA composition or integrin activation. LD2-LD3-LD4 inhibited FAK-dependent downstream integrin signaling and, unlike existing inhibitors, also blocked FAK's scaffolding functions. We further show that LD2-LD3-LD4 expression markedly reduces FA turnover and inhibits tumor cell migration and invasion. Finally, we show that dimers of a single motif, linked through a flexible linker of the proper size, are sufficient for the displacement of FAK from FAs and for inhibition of tumor cell migration. This work raises the possibility of using a synthetic peptide as an antimetastatic agent, given that effective displacement of FAK from FAs only requires dimers of a single LD motif linked by a short flexible linker. CONCLUSION: In conclusion, these results suggest that FAK displacement from FAs is a promising new strategy to target critical processes implicated in cancer progression and metastasis. Video abstract.

Recognition of LD motifs by the focal adhesion targeting domains of focal adhesion kinase and proline-rich tyrosine kinase 2-beta: Insights from molecular dynamics simulations.

The focal adhesion kinase (FAK) and the proline-rich tyrosine kinase 2-beta (PYK2) are implicated in cancer progression and metastasis and represent promising biomarkers and targets for cancer therapy. FAK and PYK2 are recruited to focal adhesions (FAs) via interactions between their FA targeting (FAT) domains and conserved segments (LD motifs) on the proteins Paxillin, Leupaxin, and Hic-5. A promising new approach for the inhibition of FAK and PYK2 targets interactions of the FAK domains with proteins that promote localization at FAs. Advances toward this goal include the development of surface plasmon resonance, heteronuclear single quantum coherence nuclear magnetic resonance (HSQC-NMR) and fluorescence polarization assays for the identification of fragments or compounds interfering with the FAK-Paxillin interaction. We have recently validated this strategy, showing that Paxillin mimicking polypeptides with 2 to 3 LD motifs displace FAK from FAs and block kinase-dependent and independent functions of FAK, including downstream integrin signaling and FA localization of the protein p130Cas. In the present work we study by all-atom molecular dynamics simulations the recognition of peptides with the Paxillin and Leupaxin LD motifs by the FAK-FAT and PYK2-FAT domains. Our simulations and free-energy analysis interpret experimental data on binding of Paxillin and Leupaxin LD motifs at FAK-FAT and PYK2-FAT binding sites, and assess the roles of consensus LD regions and flanking residues. Our results can assist in the design of effective inhibitory peptides of the FAK-FAT: Paxillin and PYK2-FAT:Leupaxin complexes and the construction of pharmacophore models for the discovery of potential small-molecule inhibitors of the FAK-FAT and PYK2-FAT focal adhesion based functions.

Addressing the Functional Determinants of FAK during Ciliogenesis in Multiciliated Cells.

We previously identified focal adhesion kinase (FAK) as an important regulator of ciliogenesis in multiciliated cells. FAK and other focal adhesion (FA) proteins associate with the basal bodies and their striated rootlets and form complexes named ciliary adhesions (CAs). CAs display similarities with FAs but are established in an integrin independent fashion and are responsible for anchoring basal bodies to the actin cytoskeleton during ciliogenesis as well as in mature multiciliated cells. FAK down-regulation leads to aberrant ciliogenesis due to impaired association between the basal bodies and the actin cytoskeleton, suggesting that FAK is an important regulator of the CA complex. However, the mechanism through which FAK functions in the complex is not clear, and in this study we examined the role of this protein in both ciliogenesis and ciliary function. We show that localization of FAK at CAs depends on interactions taking place at the amino-terminal (FERM) and carboxyl-terminal (FAT) domains and that both domains are required for proper ciliogenesis and ciliary function. Furthermore, we show that an interaction with another CA protein, paxillin, is essential for correct localization of FAK in multiciliated cells. This interaction is indispensable for both ciliogenesis and ciliary function. Finally, we provide evidence that despite the fact that FAK is in the active, open conformation at CAs, its kinase activity is dispensable for ciliogenesis and ciliary function revealing that FAK plays a scaffolding role in multiciliated cells. Overall these data show that the role of FAK at CAs displays similarities but also important differences compared with its role at FAs.

A dominant-negative provides new insights into FAK regulation and function in early embryonic morphogenesis.

FAK is a non-receptor tyrosine kinase involved in a wide variety of biological processes and crucial for embryonic development. In this manuscript, we report the generation of a new FAK dominant negative (FF), composed of the C terminus (FRNK) and the FERM domain of the protein. FF, unlike FRNK and FERM, mimics the localization of active FAK in the embryo, demonstrating that both domains are necessary to target FAK to its complexes in vivo. We show that the FERM domain has a role in the recruitment of FAK on focal adhesions and controls the dynamics of the protein on these complexes. Expression of FF blocks focal adhesion turnover and, unlike FRNK, acts as a dominant negative in vivo. FF expression in Xenopus results in an overall phenotype remarkably similar to the FAK knockout in mice, including loss of mesodermal tissues. Expression of FF in the animal cap revealed a previously unidentified role of FAK in early morphogenesis and specifically epiboly. We show that a fibronectin-derived signal transduced by FAK governs polarity and cell intercalation. Finally, failure of epiboly results in severe gastrulation problems that can be rescued by either mechanical or pharmacological relief of tension within the animal cap, demonstrating that epiboly is permissive for gastrulation. Overall, this work introduces a powerful new tool for the study of FAK, uncovers new roles for FAK in morphogenesis and reveals new mechanisms through which the FERM domain regulates the localization and dynamics of FAK.

Calpain2 protease: A new member of the Wnt/Ca(2+) pathway modulating convergent extension movements in Xenopus.

Calpains are a family of calcium-dependent intracellular cysteine proteases that regulate several physiological processes by limited cleavage of different substrates. The role of Calpain2 in embryogenesis is not clear with conflicting evidence from a number of mouse knockouts. Here we report the temporal and spatial expression of Calpain2 in Xenopus laevis embryos and address its role in Xenopus development. We show that Calpain2 is expressed maternally with elevated expression in neural tissues and that Calpain2 activity is spatially and temporally regulated. Using a Calpain inhibitor, a dominant negative and a morpholino oligonoucleotide we demonstrate that impaired Calpain2 activity results in defective convergent extension both in mesodermal and neural tissues. Specifically, Calpain2 downregulation results in loss of tissue polarity and blockage of mediolateral intercalation in Keller explants without affecting adherens junction turnover. We further show that Calpain2 is activated in response to Wnt5a and that the inhibitory effect of Wnt5a expression on animal cap elongation can be rescued by blocking Calpain2 function. This suggests that Calpain2 activity needs to be tightly regulated during convergent extension. Finally we show that expression of Xdd1 blocks the membrane translocation of Calpain2 suggesting that Calpain2 activation is downstream of Dishevelled. Overall our data show that Calpain2 activation through the Wnt/Ca(2+) pathway and Dishevelled can modulate convergent extension movements.

Xenopus laevis nucleotide binding protein 1 (xNubp1) is important for convergent extension movements and controls ciliogenesis via regulation of the actin cytoskeleton.

Nucleotide binding protein 1 (Nubp1) is a highly conserved phosphate loop (P-loop) ATPase involved in diverse processes including iron-sulfur protein assembly, centrosome duplication and lung development. Here, we report the cloning, expression and functional characterization of Xenopus laevis Nubp1. We show that xNubp1 is expressed maternally, displays elevated expression in neural tissues and is required for convergent extension movements and neural tube closure. In addition, xNubp1knockdown leads to defective ciliogenesis of the multi-ciliated cells of the epidermis as well as the monociliated cells of the gastrocoel roof plate. Specifically, xNubp1 is required for basal body migration, spacing and docking in multi-ciliated cells and basal body positioning and axoneme elongation in monociliated gastrocoel roof plate cells. Live imaging of the different pools of actin and basal body migration during the process of ciliated cell intercalation revealed that two independent pools of actin are present from the onset of cell intercalation; an internal network surrounding the basal bodies, anchoring them to the cell cortex and an apical pool of punctate actin which eventually matures into the characteristic apical actin network. We show that xNubp1 colocalizes with the apical actin network of multiciliated cells and that problems in basal body transport in xNubp1 morphants are associated with defects of the internal network of actin, while spacing and polarity issues are due to a failure of the apical and sub-apical actin pools to mature into a network. Effects of xNubp1 knockdown on the actin cytoskeleton are independent of RhoA localization and activation, suggesting that xNubp1 may have a direct role in the regulation of the actin cytoskeleton.

JNK regulates ciliogenesis through the interflagellar transport complex and actin networks.

The c-Jun N-terminal kinase (JNK) regulates various important physiological processes. Although the JNK pathway has been under intense investigation for over 20 yr, its complexity is still perplexing, with multiple protein partners underlying the diversity of its activity. We show that JNK is associated with the basal bodies in both primary and motile cilia. Loss of JNK disrupts basal body migration and docking and leads to severe ciliogenesis defects. JNK's involvement in ciliogenesis stems from a dual role in the regulation of the actin networks of multiciliated cells (MCCs) and the establishment of the intraflagellar transport-B core complex. JNK signaling is also critical for the maintenance of the actin networks and ciliary function in mature MCCs. JNK is implicated in the development of diabetes, neurodegeneration, and liver disease, all of which have been linked to ciliary dysfunction. Our work uncovers a novel role of JNK in ciliogenesis and ciliary function that could have important implications for JNK's role in the disease.

Comparing intracellular stability and targeting of sulfobetaine quantum dots with other surface chemistries in live cells.

The in vivo labeling of intracellular components with quantum dots (QDs) is very limited because of QD aggregation in the cell cytoplasm and/or QD confinement into lysosomal compartments. In order to improve intracellular targeting with QDs, various surface chemistries and delivery methods have been explored, but they have not yet been compared systematically with respect to the QD intracellular stability. In this work, the intracellular aggregation kinetics of QDs for three different surface chemistries based on ligand exchange or encapsulation with amphiphilic polymers are compared. For each surface chemistry, three delivery methods for bringing the nanoparticles into the cells are compared: electroporation, microinjection, and pinocytosis. It is concluded that the QD intracellular aggregation behavior is strongly dependent on the surface chemistry. QDs coated with dihydrolipoic acid-sulfobetaine (DHLA-SB) ligands diffuse freely in cells for longer periods of time than for QDs in the other chemistries tested, and they can access all cytoplasmic compartments. Even when conjugated to streptavidin, these DHLA-SB QDs remain freely diffusing inside the cytoplasm and unaggregated, and they are able to reach a biotinylated target inside HeLa cells. Such labeling was more efficient when compared to commercial streptavidin-conjugated QDs, which may be due to the smaller size of DHLA-SB QDs and/or to their superior intracellular stability.

High-resolution whole-mount in situ hybridization using Quantum Dot nanocrystals.

The photostability and narrow emission spectra of nanometer-scale semiconductor crystallites (QDs) make them desirable candidates for whole-mount fluorescent in situ hybridization to detect mRNA transcripts in morphologically preserved intact embryos. We describe a method for direct QD labeling of modified oligonucleotide probes through streptavidin-biotin and antibody-mediated interactions (anti-FITC and anti-digoxigenin). To overcome permeability issues and allow QD conjugate penetration, embryos were treated with proteinase K. The use of QDs dramatically increased sensitivity of whole-mount in situ hybridization (WISH) in comparison with organic fluorophores and enabled fluorescent detection of specific transcripts within cells without the use of enzymatic amplification. Therefore, this method offers significant advantages both in terms of sensitivity, as well as resolution. Specifically, the use of QDs alleviates issues of photostability and limited brightness plaguing organic fluorophores and allows fluorescent imaging of cleared embryos. It also offers new imaging possibilities, including intracellular localization of mRNAs, simultaneous multiple-transcript detection, and visualization of mRNA expression patterns in 3D.

The apical ciliary adhesion complex is established at the basal foot of motile cilia and depends on the microtubule network.

The Ciliary Adhesion (CA) complex forms in close association with the basal bodies of cilia during the early stages of ciliogenesis and is responsible for mediating complex interactions with the actin networks of multiciliated cells (MCCs). However, its precise localization with respect to basal body accessory structures and the interactions that lead to its establishment in MCCs are not well understood. Here, we studied the distribution of the CA proteins using super-resolution imaging and possible interactions with the microtubule network. The results of this study reveal that the apical CA complex forms at the distal end of the basal foot and depends on microtubules. Our data also raise the possibility that CAs may have additional roles in the regulation of the organization of the microtubule network of MCCs.

Somitic mesoderm morphogenesis is necessary for neural tube closure during Xenopus development.

Neural tube closure is a fundamental process during vertebrate embryogenesis, which leads to the formation of the central nervous system. Defective neural tube closure leads to neural tube defects which are some of the most common human birth defects. While the intrinsic morphogenetic events shaping the neuroepithelium have been studied extensively, how tissues mechanically coupled with the neural plate influence neural tube closure remains poorly understood. Here, using Xenopus laevis embryos, live imaging in combination with loss of function experiments and morphometric analysis of fixed samples we explore the reciprocal mechanical communication between the neural plate and the somitic mesoderm and its impact on tissue morphogenesis. We show that although somitic mesoderm convergent extension occurs independently from neural plate morphogenesis neural tube closure depends on somitic mesoderm morphogenesis. Specifically, impaired somitic mesoderm remodelling results in defective apical constriction within the neuroepithelium and failure of neural tube closure. Last, our data reveal that mild abnormalities in somitic mesoderm and neural plate morphogenesis have a synergistic effect during neurulation, leading to severe neural tube closure defects. Overall, our data reveal that defective morphogenesis of tissues mechanically coupled with the neural plate can not only drastically exacerbate mild neural tube defects that may arise from abnormalities within the neural tissue but can also elicit neural tube defects even when the neural plate is itself free of inherent defects.

Adherens junctions stimulate and spatially guide integrin activation and extracellular matrix deposition.

Cadherins and integrins are intrinsically linked through the actin cytoskeleton and are largely responsible for the mechanical integrity and organization of tissues. We show that cadherin clustering stimulates and spatially guides integrin activation. Adherens junction (AJ)-associated integrin activation depends on locally generated tension and does not require extracellular matrix ligands. It leads to the creation of primed integrin clusters, which spatially determine where focal adhesions will form if ligands are present and where ligands will be deposited. AJs that display integrin activation are targeted by microtubules facilitating their disassembly via caveolin-based endocytosis, showing that integrin activation impacts the stability of the core cadherin complex. Thus, the interplay between cadherins and integrins is more intimate than what was once believed and is rooted in the capacity of active integrins to be stabilized via AJ-generated tension. Altogether, our data establish a mechanism of cross-regulation between cadherins and integrins.

Histone N-terminal acetyltransferase NAA40 links one-carbon metabolism to chemoresistance.

Aberrant function of epigenetic modifiers plays an important role not only in the progression of cancer but also the development of drug resistance. N-alpha-acetyltransferase 40 (NAA40) is a highly specific epigenetic enzyme catalyzing the transfer of an acetyl moiety at the N-terminal end of histones H4 and H2A. Recent studies have illustrated the essential oncogenic role of NAA40 in various cancer types but its role in chemoresistance remains unclear. Here, using transcriptomic followed by metabolomic analysis in colorectal cancer (CRC) cells, we demonstrate that NAA40 controls key one-carbon metabolic genes and corresponding metabolites. In particular, through its acetyltransferase activity NAA40 regulates the methionine cycle thereby affecting global histone methylation and CRC cell survival. Importantly, NAA40-mediated metabolic rewiring promotes resistance of CRC cells to antimetabolite chemotherapy in vitro and in xenograft models. Specifically, NAA40 stimulates transcription of the one-carbon metabolic gene thymidylate synthase (TYMS), whose product is targeted by 5-fluorouracil (5-FU) and accordingly in primary CRC tumours NAA40 expression associates with TYMS levels and poorer 5-FU response. Mechanistically, NAA40 activates TYMS by preventing enrichment of repressive H2A/H4S1ph at the nuclear periphery. Overall, these findings define a novel regulatory link between epigenetics and cellular metabolism mediated by NAA40, which is harnessed by cancer cells to evade chemotherapy.

Single-shot optical sectioning using two-color probes in HiLo fluorescence microscopy.

We describe a wide-field fluorescence microscope setup which combines HiLo microscopy technique with the use of a two-color fluorescent probe. It allows one-shot fluorescence optical sectioning of thick biological moving sample which is illuminated simultaneously with a flat and a structured pattern at two different wavelengths. Both homogenous and structured fluorescence images are spectrally separated at detection and combined similarly with the HiLo microscopy technique. We present optically sectioned full-field images of Xenopus laevis embryos acquired at 25 images/s frame rate.

Split-inteins for simultaneous, site-specific conjugation of quantum dots to multiple protein targets in vivo.

BACKGROUND: Proteins labelled with Quantum Dots (QDs) can be imaged over long periods of time with ultrahigh spatial and temporal resolution, yielding important information on the spatiotemporal dynamics of proteins within live cells or in vivo. However one of the major problems regarding the use of QDs for biological imaging is the difficulty of targeting QDs onto proteins. We have recently developed a DnaE split intein-based method to conjugate Quantum Dots (QDs) to the C-terminus of target proteins in vivo. In this study, we expand this approach to achieve site-specific conjugation of QDs to two or more proteins simultaneously with spectrally distinguishable QDs for multiparameter imaging of cellular functions. RESULTS: Using the DnaE split intein we target QDs to the C-terminus of paxillin and show that paxillin-QD conjugates become localized at focal adhesions allowing imaging of the formation and dissolution of these complexes. We go on to utilize a different split intein, namely Ssp DnaB mini-intein, to demonstrate N-terminal protein tagging with QDs. Combination of these two intein systems allowed us to simultaneously target two distinct proteins with spectrally distinguishable QDs, in vivo, without any cross talk between the two intein systems. CONCLUSIONS: Multiple target labeling is a unique feature of the intein based methodology which sets it apart from existing tagging methodologies in that, given the large number of characterized split inteins, the number of individual targets that can be simultaneously tagged is only limited by the number of QDs that can be spectrally distinguished within the cell. Therefore, the intein-mediated approach for simultaneous, in vivo, site-specific (N- and C-terminus) conjugation of Quantum Dots to multiple protein targets opens up new possibilities for bioimaging applications and offers an effective system to target QDs and other nanostructures to intracellular compartments as well as specific molecular complexes.

Mitotic cell responses to substrate topological cues are independent of the molecular nature of adhesion.

Correct selection of the cell division axis is important for cell differentiation, tissue and organ morphogenesis, and homeostasis. Both integrins, which mediate interactions with extracellular matrix (ECM) components such as fibronectin, and cadherins, which mediate interactions between cells, are implicated in the determination of spindle orientation. We found that both cadherin- and integrin-based adhesion resulted in cell divisions parallel to the attachment plane and elicited identical spindle responses to spatial adhesive cues. This suggests that adhesion topology provides purely mechanical spatial cues that are independent of the molecular nature of the interaction or signaling from adhesion complexes. We also demonstrated that cortical integrin activation was indispensable for correct spindle orientation on both cadherin and fibronectin substrates. These data suggest that spindle orientation responses to adhesion topology are primarily a result of force anisotropy on the cell cortex and show that integrins play a central role in this process that is distinct from their role in cell-ECM interactions.

Intein-mediated site-specific conjugation of Quantum Dots to proteins in vivo.

We describe an intein based method to site-specifically conjugate Quantum Dots (QDs) to target proteins in vivo. This approach allows the covalent conjugation of any nanostructure and/or nanodevice to any protein and thus the targeting of such material to any intracellular compartment or signalling complex within the cells of the developing embryo. We genetically fused a pleckstrin-homology (PH) domain with the N-terminus half of a split intein (IN). The C-terminus half (IC) of the intein was conjugated to QDs in vitro. IC-QD's and RNA encoding PH-IN were microinjected into Xenopus embryos. In vivo intein-splicing resulted in fully functional QD-PH conjugates that could be monitored in real time within live embryos. Use of Near Infra Red (NIR)-emitting QDs allowed monitoring of QD-conjugates within the embryo at depths where EGFP is undetectable demonstrating the advantages of QD's for this type of experiment. In conclusion, we have developed a novel in vivo methodology for the site-specific conjugation of QD's and other artificial structures to target proteins in different intracellular compartments and signaling complexes.

Determining Temporal and Spatial Expression of Calpains in Amphibians.

Calpains are a family of calcium-dependent intracellular cysteine proteases that regulate important physiological processes by substrate cleavage. Despite the fact that Calpains have been identified in the Xenopus genome, their expression patterns and role have not been characterized. Therefore, herein, we describe two methods to determine temporal and spatial expression of Calpain 2 during Xenopus development, namely, RT-PCR and whole-mount in situ hybridization (WISH). In addition, indirect immunofluorescence (IF) is described to determine translocation to the plasma membrane, which correlates with activity levels of Calpain 2.

Methods of Calpain Inhibition to Determine the Role of Calpains in Embryo Development in Amphibians.

Calpains are a family of calcium-dependent intracellular cysteine proteases that regulate important physiological processes by substrate cleavage. Despite the fact that the role of calpains in cell migration and other processes has been extensively studied in vitro, the same does not apply to cell migration and morphogenetic events during embryogenesis, in vivo. Herein, we describe the use of three different methods to selectively block calpain activity in vivo in order to investigate the impact on Xenopus gastrulation and neurulation, namely, a calpain inhibitor, a dominant negative, and a morpholino antisense oligonucleotide (MO). We also provide methods to determine the effectiveness of the calpain inhibition and effect on cell fate specification and morphogenetic movements, during embryogenesis in vivo.