Daydreamer, a Ras effector and GSK-3 substrate, is important for directional sensing and cell motility

GSK3 and lamellipodin balance lamellipodial protrusions and focal adhesion maturation in mouse neural crest migration

Neural crest cells are multipotent cells that delaminate from the neuroepithelium, migrating throughout the embryo. Aberrant migration causes developmental defects. Animal models are improving our understanding of neural crest anomalies, but in vivo migration behaviors are poorly understood. Here, we demonstrate that murine neural crest cells display actin-based lamellipodia and filopodia in vivo. Using neural crest-specific knockouts or inhibitors, we show that the serine-threonine kinase glycogen synthase kinase-3 (GSK3) and the cytoskeletal regulator lamellipodin (Lpd) are required for lamellipodia formation while preventing focal adhesion maturation. Lpd is a substrate of GSK3, and phosphorylation of Lpd favors interactions with the Scar/WAVE complex (lamellipodia formation) at the expense of VASP and Mena interactions (adhesion maturation and filopodia formation). This improved understanding of cytoskeletal regulation in mammalian neural crest migration has general implications for neural crest anomalies and cancer.

Dynamic subcellular localization of DydA in Dictyostelium cells

DydA plays an important role in chemotaxis, development, and cell growth as an adaptor protein that connects Ras signaling and cytoskeletal rearrangement. DydA is a downstream effector of RasG and is involved in controlling cell polarity and pseudopodia formation during chemoattractant-directed cell migration. To understand the mechanism by which DydA functions on the cell migration, we investigated the dynamic subcellular localization of DydA in response to chemoattractant stimulation and found that DydA rapidly and transiently translocated to the cell cortex through the RA domain and the PRM region in DydA in response to chemoattractant stimulation. The PRM region appears to play a primary role in the translocation of DydA to the cell cortex and in its localization to the actin foci at the bottom of cells. Colocalization experiments of GFP-PRM with RFP-coronin indicated that GFP-PRM preceded GFP-coronin by 2-3 s in response to chemoattractant stimulation. These results suggest that the PRM region plays an indispensable role in relaying upstream regulators, such as RasG, to downstream effectors by mediating the localization of DydA to the cell cortex upon chemoattractant stimulation.

Regulation of mTORC2 Signaling

Mammalian target of rapamycin (mTOR), a serine/threonine protein kinase and a master regulator of cell growth and metabolism, forms two structurally and functionally distinct complexes, mTOR complex 1 (mTORC1) and mTORC2. While mTORC1 signaling is well characterized, mTORC2 is relatively poorly understood. mTORC2 appears to exist in functionally distinct pools, but few mTORC2 effectors/substrates have been identified. Here, we review recent advances in our understanding of mTORC2 signaling, with particular emphasis on factors that control mTORC2 activity.

Glycogen synthase kinase 3 (GSK-3) controls T-cell motility and interactions with antigen presenting cells

Objective

The threonine/serine kinase glycogen synthase kinase 3 (GSK-3) targets multiple substrates in T-cells, regulating the expression of Tbet and PD-1 on T-cells. However, it has been unclear whether GSK-3 can affect the motility of T-cells and their interactions with antigen presenting cells.

Results

Here, we show that GSK-3 controls T-cell motility and interactions with other cells. Inhibition of GSK-3, using structurally distinct inhibitors, reduced T-cell motility in terms of distance and displacement. While SB415286 reduced the number of cell-cell contacts, the dwell times of cells that established contacts with other cells did not differ for T-cells treated with SB415286. Further, the increase in cytolytic T-cell (CTL) function in killing tumor targets was not affected by the inhibition of motility. This data shows that the inhibition of GSK-3 has differential effects on T-cell motility and CTL function where the negative effects on cell–cell interactions is overridden by the increased cytolytic potential of CTLs.

Phosphodiesterase PdeD, dynacortin, and a Kelch repeat-containing protein are direct GSK3 substrates in Dictyostelium that contribute to chemotaxis towards cAMP

The migration of cells according to a diffusible chemical signal in their environment is called chemotaxis, and the slime mold Dictyostelium discoideum is widely used for the study of eukaryotic chemotaxis. Dictyostelium must sense chemicals, such as cAMP, secreted during starvation to move towards the sources of the signal. Previous work demonstrated that the gskA gene encodes the Dictyostelium homologue of glycogen synthase kinase 3 (GSK3), a highly conserved serine/threonine kinase, which plays a major role in the regulation of Dictyostelium chemotaxis. Cells lacking the GskA substrates Daydreamer and GflB exhibited chemotaxis defects less severe than those exhibited by gskA^- (GskA null) cells, suggesting that additional GskA substrates might be involved in chemotaxis. Using phosphoproteomics we identify the GskA substrates PdeD, dynacortin and SogA and characterize the phenotypes of their respective null cells in response to the chemoattractant cAMP. All three chemotaxis phenotypes are defective, and in addition, we determine that carboxylesterase D2 is a common downstream effector of GskA, its direct substrates PdeD, GflB and the kinases GlkA and YakA, and that it also contributes to cell migration. Our findings identify new GskA substrates in cAMP signalling and break down the essential role of GskA in myosin II regulation.

Phosphorylated Rho-GDP directly activates mTORC2 kinase towards AKT through dimerization with Ras-GTP to regulate cell migration

mTORC2 plays critical roles in metabolism, cell survival and actin cytoskeletal dynamics through the phosphorylation of AKT. Despite its importance to biology and medicine, it is unclear how mTORC2-mediated AKT phosphorylation is controlled. Here, we identify an unforeseen principle by which a GDP-bound form of the conserved small G protein Rho GTPase directly activates mTORC2 in AKT phosphorylation in social amoebae (Dictyostelium discoideum) cells. Using biochemical reconstitution with purified proteins, we demonstrate that Rho-GDP promotes AKT phosphorylation by assembling a supercomplex with Ras-GTP and mTORC2. This supercomplex formation is controlled by the chemoattractant-induced phosphorylation of Rho-GDP at S192 by GSK-3. Furthermore, Rho-GDP rescues defects in both mTORC2-mediated AKT phosphorylation and directed cell migration in Rho-null cells in a manner dependent on phosphorylation of S192. Thus, in contrast to the prevailing view that the GDP-bound forms of G proteins are inactive, our study reveals that mTORC2-AKT signalling is activated by Rho-GDP.

The Dictyostelium GSK3 kinase GlkA coordinates signal relay and chemotaxis in response to growth conditions

GSK3 plays a central role in orchestrating key biological signaling pathways, including cell migration. Here, we identify GlkA as a GSK3 family kinase with functions that overlap with and are distinct from those of GskA. We show that GlkA, as previously shown for GskA, regulates the cell's cytoskeleton through MyoII assembly and control of Ras and Rap1 function, leading to aberrant cell migration. However, there are both qualitative and quantitative differences in the regulation of Ras and Rap1 and their downstream effectors, including PKB, PKBR1, and PI3K, with glkA^- cells exhibiting a more severe chemotaxis phenotype than gskA^- cells. Unexpectedly, the severe glkA^- phenotypes, but not those of gskA^-, are only exhibited when cells are grown attached to a substratum but not in suspension, suggesting that GlkA functions as a key kinase of cell attachment signaling. Using proteomic iTRAQ analysis we show that there are quantitative differences in the pattern of protein expression depending on the growth conditions in wild-type cells. We find that GlkA expression affects the cell's proteome during vegetative growth and development, with many of these changes depending on whether the cells are grown attached to a substratum or in suspension. These changes include key cytoskeletal and signaling proteins known to be essential for proper chemotaxis and signal relay during the aggregation stage of Dictyostelium development.

A Gα-Stimulated RapGEF Is a Receptor-Proximal Regulator of Dictyostelium Chemotaxis

Chemotaxis, or directional movement toward extracellular chemical gradients, is an important property of cells that is mediated through G-protein-coupled receptors (GPCRs). Although many chemotaxis pathways downstream of Gβγ have been identified, few Gα effectors are known. Gα effectors are of particular importance because they allow the cell to distinguish signals downstream of distinct chemoattractant GPCRs. Here we identify GflB, a Gα2 binding partner that directly couples the Dictyostelium cyclic AMP GPCR to Rap1. GflB localizes to the leading edge and functions as a Gα-stimulated, Rap1-specific guanine nucleotide exchange factor required to balance Ras and Rap signaling. The kinetics of GflB translocation are fine-tuned by GSK-3 phosphorylation. Cells lacking GflB display impaired Rap1/Ras signaling and actin and myosin dynamics, resulting in defective chemotaxis. Our observations demonstrate that GflB is an essential upstream regulator of chemoattractant-mediated cell polarity and cytoskeletal reorganization functioning to directly link Gα activation to monomeric G-protein signaling.

The small GTPases Ras and Rap1 bind to and control TORC2 activity

Target of Rapamycin Complex 2 (TORC2) has conserved roles in regulating cytoskeleton dynamics and cell migration and has been linked to cancer metastasis. However, little is known about the mechanisms regulating TORC2 activity and function in any system. In Dictyostelium, TORC2 functions at the front of migrating cells downstream of the Ras protein RasC, controlling F-actin dynamics and cAMP production. Here, we report the identification of the small GTPase Rap1 as a conserved binding partner of the TORC2 component RIP3/SIN1, and that Rap1 positively regulates the RasC-mediated activation of TORC2 in Dictyostelium. Moreover, we show that active RasC binds to the catalytic domain of TOR, suggesting a mechanism of TORC2 activation that is similar to Rheb activation of TOR complex 1. Dual Ras/Rap1 regulation of TORC2 may allow for integration of Ras and Rap1 signaling pathways in directed cell migration.

Direct Interaction between TalinB and Rap1 is necessary for adhesion of Dictyostelium cells

Background

The small G-protein Rap1 is an important regulator of cellular adhesion in Dictyostelium, however so far the downstream signalling pathways for cell adhesion are not completely characterized. In mammalian cells talin is crucial for adhesion and Rap1 was shown to be a key regulator of talin signalling.

Results

In a proteomic screen we identified TalinB as a potential Rap1 effector in Dictyostelium. In subsequent pull-down experiments we demonstrate that the Ras association (RA) domain of TalinB interacts specifically with active Rap1. Studies with a mutated RA domain revealed that the RA domain is essential for TalinB-Rap1 interaction, and that this interaction contributes to cell-substrate adhesion during single-celled growth and is crucial for cell-cell adhesion during multicellular development.

Conclusions

Dictyostelium Rap1 directly binds to TalinB via the conserved RA domain. This interaction is critical for adhesion, which becomes essential for high adhesive force demanding processes, like morphogenesis during multicellular development of Dictyostelium. In mammalian cells the established Rap1-talin interaction is indirect and acts through the scaffold protein - RIAM. Interestingly, direct binding of mouse Rap1 to the RA domain of Talin1 has recently been demonstrated.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-015-0078-0) contains supplementary material, which is available to authorized users.

PP2A/B56 and GSK3/Ras suppress PKB activity during Dictyostelium chemotaxis

We have previously shown that the Dictyostelium protein phosphatase 2A regulatory subunit B56, encoded by psrA, modulates Dictyostelium cell differentiation through negatively affecting glycogen synthase kinase 3 (GSK3) function. Our follow-up research uncovered that B56 preferentially associated with GDP forms of RasC and RasD, but not with RasG in vitro, and psrA(-) cells displayed inefficient activation of multiple Ras species, decreased random motility, and inefficient chemotaxis toward cAMP and folic acid gradient. Surprisingly, psrA(-) cells displayed aberrantly high basal and poststimulus phosphorylation of Dictyostelium protein kinase B (PKB) kinase family member PKBR1 and PKB substrates. Expression of constitutively active Ras mutants or inhibition of GSK3 in psrA(-) cells increased activities of both PKBR1 and PKBA, but only the PKBR1 activity was increased in wild-type cells under the equivalent conditions, indicating that either B56- or GSK3-mediated suppressive mechanism is sufficient to maintain low PKBA activity, but both mechanisms are necessary for suppressing PKBR1. Finally, cells lacking RasD or RasC displayed normal PKBR1 regulation under GSK3-inhibiting conditions, indicating that RasC or RasD proteins are essential for GSK3-mediated PKBR1 inhibition. In summary, B56 constitutes inhibitory circuits for PKBA and PKBR1 and thus heavily affects Dictyostelium chemotaxis.

Moving towards a paradigm: common mechanisms of chemotactic signaling in Dictyostelium and mammalian leukocytes

Chemotaxis, or directed migration of cells along a chemical gradient, is a highly coordinated process that involves gradient sensing, motility, and polarity. Most of our understanding of chemotaxis comes from studies of cells undergoing amoeboid-type migration, in particular the social amoeba Dictyostelium discoideum and leukocytes. In these amoeboid cells the molecular events leading to directed migration can be conceptually divided into four interacting networks: receptor/G protein, signal transduction, cytoskeleton, and polarity. The signal transduction network occupies a central position in this scheme as it receives direct input from the receptor/G protein network, as well as feedback from the cytoskeletal and polarity networks. Multiple overlapping modules within the signal transduction network transmit the signals to the actin cytoskeleton network leading to biased pseudopod protrusion in the direction of the gradient. The overall architecture of the networks, as well as the individual signaling modules, is remarkably conserved between Dictyostelium and mammalian leukocytes, and the similarities and differences between the two systems are the subject of this review.

Phospho-NHE3 forms membrane patches and interacts with beta-actin to sense and maintain constant direction during cell migration

The Na(+)/H(+) exchanger NHE3 colocalizes with beta-actin at the leading edge of directionally migrating cells. Using human osteosarcoma cells (SaOS-2), rat osteoblasts (calvaria), and human embryonic kidney (HEK) cells, we identified a novel role for NHE3 via beta-actin in anode and cathode directed motility, during electrotaxis. NHE3 knockdown by RNAi revealed that NHE3 expression is required to achieve constant directionality and polarity in migrating cells. Phosphorylated NHE3 (pNHE3) and beta-actin complex formation was impaired by the NHE3 inhibitor S3226 (IC50 0.02µM). Fluorescence cross-correlation spectroscopy (FCCS) revealed that the molecular interactions between NHE3 and beta-actin in membrane protrusions increased 1.7-fold in the presence of a directional cue and decreased 3.3-fold in the presence of cytochalasin D. Data from flow cytometric analysis showed that membrane potential of cells (Vmem) decreases in directionally migrating, NHE3-deficient osteoblasts and osteosarcoma cells whereas only Vmem of wild type osteoblasts is affected during directional migration. These findings suggest that pNHE3 has a mechanical function via beta-actin that is dependent on its physiological activity and Vmem. Furthermore, phosphatidylinositol 3,4,5-trisphosphate (PIP3) levels increase while PIP2 remains stable when cells have persistent directionality. Both PI3 kinase (PI3K) and Akt expression levels change proportionally to NHE3 levels. Interestingly, however, the content of pNHE3 level does not change when PI3K/Akt is inhibited. Therefore, we conclude that NHE3 can act as a direction sensor for cells and that NHE3 phosphorylation in persistent directional cell migration does not involve PI3K/Akt during electrotaxis.

Glycogen Synthase Kinase 3 influences cell motility and chemotaxis by regulating PI3K membrane localization in Dictyostelium

Glycogen Synthase Kinase 3 (GSK3) is a multifunctional kinase involved in diverse cellular activities such as metabolism, differentiation, and morphogenesis. Recent studies showed that GSK3 in Dictyostelium affects chemotaxis via TorC2 pathway and Daydreamer. Now we report that GSK3 affects PI3K membrane localization, of which the mechanism has remained to be fully understood in Dictyostelium. The membrane localization domain (LD) of Phosphatidylinositol-3-kinase 1 (PI3K1) is phosphorylated on serine residues in a GSK3 dependent mechanism and PI3K1-LD exhibited biased membrane localization in gsk3(-) cells compared to the wild type cells. Furthermore, multiple GSK3-phosphorylation consensus sites exist in PI3K1-LD, of which phosphomimetic substitutions restored cAMP induced transient membrane localization of PI3K1-LD in gsk3(-) cells. Serine to alanine substitution mutants of PI3K1-LD, in contrast, displayed constitutive membrane localization in wild type cells. Biochemical analysis revealed that GSK3 dependent serine phosphorylation of PI3K1-LD is constitutive during the course of cAMP stimulation. Together, these data suggest that GSK3 dependent serine phosphorylation is a prerequisite for chemoattractant cAMP induced PI3K membrane localization.