Regulation of the nuclear speckle localization and function of Rac1.

Despite significant evidence that Rac1 is localized to the nucleus, little is known regarding the function and biological significance of nuclear Rac1. Here, we showed that in response to EGF Rac1 was translocated to nuclear speckles and co-localized with the nuclear speckle marker Serine/arginine-rich splicing factor 2 (SRSF2) in Cos-7 cells. We also showed that the nuclear speckle localization of Rac1 was dependent on its T108 phosphorylation and facilitated by Rac1 polybasic region (PBR) that contains a nuclear localization signal and Rac1 GTPase activity. To gain insight into the function of Rac1 in nuclear speckles, we searched for Rac1 binding proteins in the nucleus. We isolated nuclear fraction of HEK 293 cells and incubated with GST-Rac1 and the phosphomimetic GST-Rac1T108E. We identified 463 proteins that were associated with GST-Rac1T108E, but not with GST-Rac1 by LC-MS/MS. Three notable groups of these proteins are: the heterogeneous nuclear ribonucleoproteins (hnRNPs), small nuclear ribonucleoproteins (snRNPs), and SRSFs, all of which are involved in pre-mRNA splicing and associated with nuclear speckles. We further showed by co-immunoprecipitation that Rac1 interacts with SRSF2, hnRNPA1, and U2A' in response to EGF. The interaction is dependent on T108 phosphorylation and facilitated by Rac1 PBR and GTPase activity. We showed that hnRNPA1 translocated in and out of nucleus in response to EGF in a similar pattern to Rac1. Rac1 only partially colocalized with U2A' that localizes to the actual splicing sites adjacent to nuclear speckle. Finally, we showed that Rac1 regulated EGF-induced pre-mRNA splicing and this is mediated by T108 phosphorylation. We conclude that in response to EGF, T108 phosphorylated Rac1 is targeted to nuclear speckles, interacts with multiple groups of proteins involved in pre-mRNA splicing, and regulates EGF-induced pre-mRNA splicing.

Differential Subcellular Distribution and Translocation of Seven 14-3-3 Isoforms in Response to EGF and During the Cell Cycle.

Multiple isoforms of 14-3-3 proteins exist in different organisms. In mammalian cells, 14-3-3 protein has seven isoforms (α/ß, ε, η, γ, σ, θ/τ, and δ/ζ), with α and δ representing the phosphorylated versions of ß and ζ, respectively. While the existence of multiple isoforms may represent one more level of regulation in 14-3-3 signaling, our knowledge regarding the isoform-specific functions of 14-3-3 proteins is very limited. Determination of the subcellular localization of the different 14-3-3 isoforms could give us important clues of their specific functions. In this study, by using indirect immunofluorescence, subcellular fractionation, and immunoblotting, we studied the subcellular localization of the total 14-3-3 protein and each of the seven 14-3-3 isoforms; their redistribution throughout the cell cycle; and their translocation in response to EGF in Cos-7 cells. We showed that 14-3-3 proteins are broadly distributed throughout the cell and associated with many subcellular structures/organelles, including the plasma membrane (PM), mitochondria, ER, nucleus, microtubules, and actin fibers. This broad distribution underlines the multiple functions identified for 14-3-3 proteins. The different isoforms of 14-3-3 proteins have distinctive subcellular localizations, which suggest their distinctive cellular functions. Most notably, 14-3-3ƞ is almost exclusively localized to the mitochondria, 14-3-3γ is only localized to the nucleus, and 14-3-3σ strongly and specifically associated with the centrosome during mitosis. We also examined the subcellular localization of the seven 14-3-3 isoforms in other cells, including HEK-293, MDA-MB-231, and MCF-7 cells, which largely confirmed our findings with Cos-7 cells.

Identification of druggable regulators of cell secretion via a kinome-wide screen and high-throughput immunomagnetic cell sorting.

The identification of genetic regulators of cell secretions is challenging because it requires the sorting of a large number of cells according to their secretion patterns. Here we report the development and applicability of a high-throughput microfluidic method for the analysis of the secretion levels of large populations of immune cells. The method is linked with a kinome-wide loss-of-function CRISPR screen, immunomagnetically sorting the cells according to their secretion levels, and the sequencing of their genomes to identify key genetic modifiers of cell secretion. We used the method, which we validated against flow cytometry for cytokines secreted from primary mouse CD4+ (cluster of differentiation 4-positive) T cells, to discover a subgroup of highly co-expressed kinase-coding genes that regulate interferon-gamma secretion by these cells. We validated the function of the kinases identified using RNA interference, CRISPR knockouts and kinase inhibitors and confirmed the druggability of selected kinases via the administration of a kinase inhibitor in an animal model of colitis. The technique may facilitate the discovery of regulatory mechanisms for immune-cell activation and of therapeutic targets for autoimmune diseases.

nuPRISM: Microfluidic Genome-Wide Phenotypic Screening Platform for Cellular Nuclei.

Genome-wide loss-of-function screens are critical tools to identify novel genetic regulators of intracellular proteins. However, studying the changes in the organelle-specific expression profile of intracellular proteins can be challenging due to protein localization differences across the whole cell, hindering context-dependent protein expression and activity analyses. Here, we describe nuPRISM, a microfluidics chip specifically designed for large-scale isolated nuclei sorting. The new device enables rapid genome-wide loss-of-function phenotypic CRISPR-Cas9 screens directed at intranuclear targets. We deployed this technology to identify novel genetic regulators of ß-catenin nuclear accumulation, a phenotypic hallmark of APC-mutated colorectal cancer. nuPRISM expands our ability to capture aberrant nuclear morphological and functional traits associated with distinctive signal transduction and subcellular localization-driven functional processes with substantial resolution and high throughput.

Ultrasound-assisted magnetic nanoparticle-based gene delivery.

Targeted gene delivery is important in biomedical research and applications. In this paper, we synergistically combine non-viral chemical materials, magnetic nanoparticles (MNPs), and a physical technique, low-intensity pulsed ultrasound (LIPUS), to achieve efficient and targeted gene delivery. The MNPs are iron oxide super-paramagnetic nanoparticles, coated with polyethyleneimine (PEI), which makes a high positive surface charge and is favorable for the binding of genetic materials. Due to the paramagnetic properties of the MNPs, the application of an external magnetic field increases transfection efficiency while LIPUS stimulation enhances cell viability and permeability. We found that stimulation at the intensity of 30 mW/cm2 for 10 minutes yields optimal results with a minimal adverse effect on the cells. By combining the effect of the external magnetic field and LIPUS, the genetic material (GFP or Cherry Red plasmid) can enter the cells. The flow cytometry results showed that by using just a magnetic field to direct the genetic material, the transfection efficiency on HEK 293 cells that were treated by our MNPs was 56.1%. Coupled with LIPUS stimulation, it increased to 61.5% or 19% higher than the positive control (Lipofectamine 2000). Besides, compared with the positive control, our method showed less toxicity. Cell viability after transfection was 63.61%, which is 19% higher than the standard transfection technique. In conclusion, we designed a new gene-delivery method that is affordable, targeted, shows low-toxicity, yet high transfection efficiency, compared to other conventional approaches.

Rac1 S71 Mediates the Interaction between Rac1 and 14-3-3 Proteins.

Both 14-3-3 proteins (14-3-3s) and Rho proteins regulate cytoskeleton remodeling and cell migration, which suggests a possible interaction between the signaling pathways regulated by these two groups of proteins. Indeed, more and more emerging evidence indicates the mutual regulation of these two signaling pathways. However, all of the data regarding the interaction between Rac1 signaling pathways and 14-3-3 signaling pathways are through either the upstream regulators or downstream substrates. It is not clear if Rac1 could interact with 14-3-3s directly. It is interesting to notice that the Rac1 sequence 68RPLSYP73 is likely a 14-3-3 protein binding motif following the phosphorylation of S71 by Akt. Thus, we hypothesize that Rac1 directly interacts with 14-3-3s. We tested this hypothesis in this research. By using mutagenesis, co-immunoprecipitation (co-IP), Rac1 activity assay, immunoblotting, and indirect immunofluorescence, we demonstrate that 14-3-3s interact with Rac1. This interaction is mediated by Rac1 S71 in both phosphorylation-dependent and -independent manners, but the phosphorylation-dependent interaction is much stronger. Epidermal growth factor (EGF) strongly stimulates the phosphorylation of Rac1 S71 and the interaction between 14-3-3s and Rac1. Mutating S71 to A completely abolishes both phosphorylation-dependent and -independent interactions between 14-3-3s and Rac1. The interaction between 14-3-3s and Rac1 mostly serve to regulate the activity and subcellular localization of Rac1. Among the seven 14-3-3 isoforms, 14-3-3η, -σ, and -θ showed interactions with Rac1 in both Cos-7 and HEK 293 cells. 14-3-3γ also binds to Rac1 in HEK 293 cells, but not in Cos-7 cells. We conclude that 14-3-3s interact with Rac1. This interaction is mediated by Rac1 S71 in both phosphorylation-dependent and -independent manners. The interaction between 14-3-3 and Rac1 mostly serves to regulate the activity and subcellular localization of Rac1. Among the seven 14-3-3 isoforms, 14-3-3η, -γ, -σ, and -θ interact with Rac1.

Post-Translational Modification and Subcellular Distribution of Rac1: An Update.

Rac1 is a small GTPase that belongs to the Rho family. The Rho family of small GTPases is a subfamily of the Ras superfamily. The Rho family of GTPases mediate a plethora of cellular effects, including regulation of cytoarchitecture, cell size, cell adhesion, cell polarity, cell motility, proliferation, apoptosis/survival, and membrane trafficking. The cycling of Rac1 between the GTP (guanosine triphosphate)- and GDP (guanosine diphosphate)-bound states is essential for effective signal flow to elicit downstream biological functions. The cycle between inactive and active forms is controlled by three classes of regulatory proteins: Guanine nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs), and guanine-nucleotide-dissociation inhibitors (GDIs). Other modifications include RNA splicing and microRNAs; various post-translational modifications have also been shown to regulate the activity and function of Rac1. The reported post-translational modifications include lipidation, ubiquitination, phosphorylation, and adenylylation, which have all been shown to play important roles in the regulation of Rac1 and other Rho GTPases. Moreover, the Rac1 activity and function are regulated by its subcellular distribution and translocation. This review focused on the most recent progress in Rac1 research, especially in the area of post-translational modification and subcellular distribution and translocation.