WAVE2 forms a complex with PKA and is involved in PKA enhancement of membrane protrusions

Protein Kinase A is a Functional Component of Focal Adhesions

Focal adhesions (FAs) form the junction between extracellular matrix (ECM)-bound integrins and the actin cytoskeleton and also transmit signals that regulate cell adhesion, cytoskeletal dynamics, and cell migration. While many of these signals are rooted in reversible tyrosine phosphorylation, phosphorylation of FA proteins on Ser/Thr residues is far more abundant yet its mechanisms and consequences are far less understood. The cAMP-dependent protein kinase (protein kinase A; PKA) has important roles in cell adhesion and cell migration and is both an effector and regulator of integrin-mediated adhesion to the ECM. Importantly, subcellular localization plays a critically important role in specifying PKA function. Here, we show that PKA is present in isolated FA-cytoskeleton complexes and active within FAs in live cells. Furthermore, using kinase-catalyzed biotinylation of isolated FA-cytoskeleton complexes, we identify fifty-three high-stringency candidate PKA substrates within FAs. From this list, we validate tensin-3 (Tns3) - a well-established molecular scaffold, regulator of cell migration, and component of focal and fibrillar adhesions - as a novel direct substrate for PKA. These observations identify a new pathway for phospho-regulation of Tns3 and, importantly, establish a new and important niche for localized PKA signaling and thus provide a foundation for further investigation of the role of PKA in the regulation of FA dynamics and signaling.

Functional and structural insights into the multi-step activation and catalytic mechanism of bacterial ExoY nucleotidyl cyclase toxins bound to actin-profilin

ExoY virulence factors are members of a family of bacterial nucleotidyl cyclases (NCs) that are activated by specific eukaryotic cofactors and overproduce cyclic purine and pyrimidine nucleotides in host cells. ExoYs act as actin-activated NC toxins. Here, we explore the Vibrio nigripulchritudo Multifunctional-Autoprocessing Repeats-in-ToXin (MARTX) ExoY effector domain (Vn-ExoY) as a model for ExoY-type members that interact with monomeric (G-actin) instead of filamentous (F-actin) actin. Vn-ExoY exhibits moderate binding affinity to free or profilin-bound G-actin but can capture the G-actin:profilin complex, preventing its spontaneous or VASP- or formin-mediated assembly at F-actin barbed ends in vitro. This mechanism may prolong the activated cofactor-bound state of Vn-ExoY at sites of active actin cytoskeleton remodelling. We present a series of high-resolution crystal structures of nucleotide-free, 3'-deoxy-ATP- or 3'-deoxy-CTP-bound Vn-ExoY, activated by free or profilin-bound G-actin-ATP/-ADP, revealing that the cofactor only partially stabilises the nucleotide-binding pocket (NBP) of NC toxins. Substrate binding induces a large, previously-unidentified, closure of their NBP, confining catalytically important residues and metal cofactors around the substrate, and facilitating the recruitment of two metal ions to tightly coordinate the triphosphate moiety of purine or pyrimidine nucleotide substrates. We validate critical residues for both the purinyl and pyrimidinyl cyclase activity of NC toxins in Vn-ExoY and its distantly-related ExoY from Pseudomonas aeruginosa, which specifically interacts with F-actin. The data conclusively demonstrate that NC toxins employ a similar two-metal-ion mechanism for catalysing the cyclisation of nucleotides of different sizes. These structural insights into the dynamics of the actin-binding interface of actin-activated ExoYs and the multi-step activation of all NC toxins offer new perspectives for the specific inhibition of class II bacterial NC enzymes.

Orchestration of synaptic functions by WAVE regulatory complex-mediated actin reorganization

The WAVE regulatory complex (WRC), composed of five components-Cyfip1/Sra1, WAVE/Scar, Abi, Nap1/Nckap1, and Brk1/HSPC300-is essential for proper actin cytoskeletal dynamics and remodeling in eukaryotic cells, likely by matching various patterned signals to Arp2/3-mediated actin nucleation. Accumulating evidence from recent studies has revealed diverse functions of the WRC in neurons, demonstrating its crucial role in dictating the assembly of molecular complexes for the patterning of various trans-synaptic signals. In this review, we discuss recent exciting findings on the physiological role of the WRC in regulating synaptic properties and highlight the involvement of WRC dysfunction in various brain disorders.

The WAVE2/miR-29/Integrin-β1 Oncogenic Signaling Axis Promotes Tumor Growth and Metastasis in Triple-negative Breast Cancer

Breast cancer is the most frequently diagnosed malignancy in women and the major cause of death because of its invasion, metastasis, and resistance to therapies capabilities. The most aggressive subtype of breast cancer is triple-negative breast cancer (TNBC) due to invasive and metastatic properties along with early age of diagnosis and poor prognosis. TNBC tumors do not express estrogen, progesterone, and HER2 receptors, which limits their treatment with targeted therapies. Cancer invasiveness and metastasis are known to be promoted by increased cell motility and upregulation of the WAVE proteins. While the contribution of WAVE2 to cancer progression is well documented, the WAVE2-mediated regulation of TNBC oncogenic properties is still under investigated, as does the molecular mechanisms by which WAVE2 regulates such oncogenic pathways. In this study, we show that WAVE2 plays a significant role in TNBC development, progression, and metastasis, through the regulation of miR-29 expression, which in turn targets Integrin-β1 (ITGB1) and its downstream oncogenic activities. Conversely, we found WAVE2 expression to be regulated by miR-29 in a negative regulatory feedback loop. Reexpression of exogenous WAVE2 in the WAVE2-deficient TNBC cells resulted in reactivation of ITGB1 expression and activity, further confirming the specificity of WAVE2 in regulating Integrin-β1. Together, our data identify a novel WAVE2/miR-29/ITGB1 signaling axis, which is essential for the regulation of the invasion-metastasis cascade in TNBC. Our findings offer new therapeutic strategies for the treatment of TNBC by targeting WAVE2 and/or its downstream effectors.

Significance

Identification of a novel WAVE2/miR-29/ITGB1 signaling axis may provide new insights on how WAVE2 regulates the invasion-metastasis cascade of TNBC tumors through the modulation of ITGB1 and miR-29.

WASP family proteins: Molecular mechanisms and implications in human disease

Proteins of the Wiskott-Aldrich syndrome protein (WASP) family play a central role in regulating actin cytoskeletal dynamics in a wide range of cellular processes. Genetic mutations or misregulation of these proteins are tightly associated with many diseases. The WASP-family proteins act by transmitting various upstream signals to their conserved WH2-Central-Acidic (WCA) peptide sequence at the C-terminus, which in turn binds to the Arp2/3 complex to stimulate the formation of branched actin networks at membranes. Despite this common feature, the regulatory mechanisms and cellular functions of distinct WASP-family proteins are very different. Here, we summarize and clarify our current understanding of WASP-family proteins and how disruption of their functions is related to human disease.

The Role of WAVE2 Signaling in Cancer

The Wiskott–Aldrich syndrome protein (WASP) and WASP family verprolin-homologous protein (WAVE)—WAVE1, WAVE2 and WAVE3 regulate rapid reorganization of cortical actin filaments and have been shown to form a key link between small GTPases and the actin cytoskeleton. Upon receiving upstream signals from Rho-family GTPases, the WASP and WAVE family proteins play a significant role in polymerization of actin cytoskeleton through activation of actin-related protein 2/3 complex (Arp2/3). The Arp2/3 complex, once activated, forms actin-based membrane protrusions essential for cell migration and cancer cell invasion. Thus, by activation of Arp2/3 complex, the WAVE and WASP family proteins, as part of the WAVE regulatory complex (WRC), have been shown to play a critical role in cancer cell invasion and metastasis, drawing significant research interest over recent years. Several studies have highlighted the potential for targeting the genes encoding either part of or a complete protein from the WASP/WAVE family as therapeutic strategies for preventing the invasion and metastasis of cancer cells. WAVE2 is well documented to be associated with the pathogenesis of several human cancers, including lung, liver, pancreatic, prostate, colorectal and breast cancer, as well as other hematologic malignancies. This review focuses mainly on the role of WAVE2 in the development, invasion and metastasis of different types of cancer. This review also summarizes the molecular mechanisms that regulate the activity of WAVE2, as well as those oncogenic pathways that are regulated by WAVE2 to promote the cancer phenotype. Finally, we discuss potential therapeutic strategies that target WAVE2 or the WAVE regulatory complex, aimed at preventing or inhibiting cancer invasion and metastasis.

WAVE1 and WAVE2 have distinct and overlapping roles in controlling actin assembly at the leading edge

SCAR/WAVE proteins and Arp2/3 complex assemble branched actin networks at the leading edge. Two isoforms of SCAR/WAVE, WAVE1 and WAVE2, reside at the leading edge, yet it has remained unclear whether they perform similar or distinct roles. Further, there have been conflicting reports about the Arp2/3-independent biochemical activities of WAVE1 on actin filament elongation. To investigate this in vivo, we knocked out WAVE1 and WAVE2 genes, individually and together, in B16-F1 melanoma cells. We demonstrate that WAVE1 and WAVE2 are redundant for lamellipodia formation and motility. However, there is a significant decrease in the rate of leading edge actin extension in WAVE2 KO cells, and an increase in WAVE1 KO cells. The faster rates of actin extension in WAVE1 KO cells are offset by faster retrograde flow, and therefore do not translate into faster lamellipodium protrusion. Thus, WAVE1 restricts the rate of actin extension at the leading edge, and appears to couple actin networks to the membrane to drive protrusion. Overall, these results suggest that WAVE1 and WAVE2 have redundant roles in promoting Arp2/3-dependent actin nucleation and lamellipodia formation, but distinct roles in controlling actin network extension and harnessing network growth to cell protrusion.

Vinexin family (SORBS) proteins play different roles in stiffness-sensing and contractile force generation

Vinexin, c-Cbl associated protein (CAP) and Arg-binding protein 2 (ArgBP2) constitute an adaptor protein family called the vinexin (SORBS) family that is targeted to focal adhesions (FAs). Although numerous studies have focused on each of the SORBS proteins and partially elucidated their involvement in mechanotransduction, a comparative analysis of their function has not been well addressed. Here, we established mouse embryonic fibroblasts that individually expressed SORBS proteins and analysed their functions in an identical cell context. Both vinexin-α and CAP co-localized with vinculin at FAs and promoted the appearance of vinculin-rich FAs, whereas ArgBP2 co-localized with α-actinin at the proximal end of FAs and punctate structures on actin stress fibers (SFs), and induced paxillin-rich FAs. Furthermore, both vinexin-α and CAP contributed to extracellular matrix stiffness-dependent vinculin behaviors, while ArgBP2 stabilized α-actinin on SFs and enhanced intracellular contractile forces. These results demonstrate the differential roles of SORBS proteins in mechanotransduction.

Confinement Sensing and Signal Optimization via Piezo1/PKA and Myosin II Pathways

Cells adopt distinct signaling pathways to optimize cell locomotion in different physical microenvironments. However, the underlying mechanism that enables cells to sense and respond to physical confinement is unknown. Using microfabricated devices and substrate-printing methods along with FRET-based biosensors, we report that, as cells transition from unconfined to confined spaces, intracellular Ca²⁺ level is increased, leading to phosphodiesterase 1 (PDE1)-dependent suppression of PKA activity. This Ca²⁺ elevation requires Piezo1, a stretch-activated cation channel. Moreover, differential regulation of PKA and cell stiffness in unconfined versus confined cells is abrogated by dual, but not individual, inhibition of Piezo1 and myosin II, indicating that these proteins can independently mediate confinement sensing. Signals activated by Piezo1 and myosin II in response to confinement both feed into a signaling circuit that optimizes cell motility. This study provides a mechanism by which confinement-induced signaling enables cells to sense and adapt to different physical microenvironments.

miRNAs 182 and 183 are necessary to maintain adult cone photoreceptor outer segments and visual function

The outer segments of cones serve as light detectors for daylight color vision, and their dysfunction leads to human blindness conditions. We show that the cone-specific disruption of DGCR8 in adult mice led to the loss of miRNAs and the loss of outer segments, resulting in photoreceptors with significantly reduced light responses. However, the number of cones remained unchanged. The loss of the outer segments occurred gradually over 1 month, and during this time the genetic signature of cones decreased. Reexpression of the sensory-cell-specific miR-182 and miR-183 prevented outer segment loss. These miRNAs were also necessary and sufficient for the formation of inner segments, connecting cilia and short outer segments, as well as light responses in stem-cell-derived retinal cultures. Our results show that miR-182- and miR-183-regulated pathways are necessary for cone outer segment maintenance in vivo and functional outer segment formation in vitro.

Role of nucleation-promoting factors in mouse early embryo development

During mitosis nucleation-promoting factors (NPFs) bind to the Arp2/3 complex and activate actin assembly. JMY and WAVE2 are two critical members of the NPFs. Previous studies have demonstrated that NPFs promote multiple processes such as cell migration and cytokinesis. However, the role of NPFs in development of mammalian embryos is still unknown. Results of the present study show that the NPFs JMY and WAVE2 are critical for cytokinesis during development of mouse embryos. Both JMY and WAVE2 are expressed in mouse embryos. After injection of JMY or WAVE2 siRNA, all embryos failed to develop to the morula or blastocyst stages. Moreover, using fluorescence intensity analysis, we found that the expression of actin decreased, and multiple nuclei were observed within a single cell indicating that NPFs-induced actin reduction caused the failure of cell division. In addition, injection of JMY and WAVE2 siRNA also caused ARP2 degradation, indicating that involvement of NPFs in development of mouse embryos is mainly through regulation of ARP2/3-induced actin assembly. Taken together, these data suggested that WAVE2 and JMY are involved in development of mouse embryos, and their regulation may be through a NPFs-Arp2/3-actin pathway.

Dlg5 interacts with the TGF-β receptor and promotes its degradation

Discs large homolog 5 (Dlg5) is a member of the membrane-associated guanylate kinase adaptor family of proteins and is involved in epithelial-to-mesenchymal transition via transforming growth factor-β (TGF-β) signaling. However, the mechanism underlying the regulation of TGF-β signaling is unclear. We show here that Dlg5 interacts and colocalizes with both TGF-β type I (TβRI) and type II (TβRII) receptors at the plasma membrane. TβRI activation is not required for this interaction. Furthermore, the overexpression of Dlg5 enhances the degradation of TβRI. Proteasome inhibitors inhibited this enhanced degradation. These results suggest that Dlg5 interacts with TβRs and promotes their degradation.

Wasf2: a novel target of intermittent parathyroid hormone administration

Systemic intermittent administration of parathyroid hormone (PTH) stimulates bone formation in animals and humans, and recombinant human PTH1-34 (teriparatide) is used clinically for the treatment of osteoporosis. In this study, we investigated the regulation of gene expression by intermittent PTH administration in MC3T3-E1 osteoblastic cells. We found that intermittent PTH1-34 administration downregulated Wiskott-Aldrich syndrome protein family member (Wasf) 2 mRNA expression. Wnt inhibitor, IWP-2, and protein kinase C inhibitor, Go6976, inhibited this downregulation. However, continuous PTH did not affect Wasf2 expression. Transfection of Wasf2 siRNA reduced bone sialoprotein (BSP) mRNA expression in a similar manner following intermittent PTH administration in MC3T3-E1 cells. These results identify Wasf2 as a novel target of intermittent PTH administration via the Wnt and phosphoinositide-dependent protein kinase signaling pathways, and the resulting regulation of BSP expression may contribute to the anabolic effects of PTH.

Phosphoregulation of the WAVE regulatory complex and signal integration

The WAVE2 regulatory complex (WRC) induces actin polymerization by activating the actin nucleator Arp2/3. Polymerizing actin pushes against the cell membrane and induces dramatic edge protrusions. In order to properly control such changes in cell morphology and function, cells have evolved multiple methods to tightly regulate WRC and Arp2/3 activity in space and time. Of these mechanisms, phosphorylation plays a fundamental role in transmitting extracellular and intracellular signals to the WRC and the actin cytoskeleton. This review discusses the phosphorylation-based regulatory inputs into the WRC. Signaling pathways that respond to growth factors, chemokines, hormones, and extracellular matrix converge upon the WAVE and ABI components of the WRC. The Abl, Src, ERK, and PKA kinases promote complex activation through a WRC conformation change that permits interaction with the Arp2/3 complex and through WRC translocation to the cell edge. The neuron-specific CDK5 and constitutively active CK2 kinases inhibit WRC activation. These regulatory signals are integrated in space and time as they coalesce upon the WRC. The combination of WRC phosphorylation events and WRC activity is controlled by stimulus, cell type, and cell cycle-specific pathway activation and via pathway cross-inhibition and cross-activation.

Carbonic anhydrase IX: a hypoxia-controlled "catalyst" of cell migration

Cell migration can be principally viewed as a chain of well-orchestrated morphological events that lead to dynamic reshaping of the cell body. However, behind the scene of such a "morphological theater" there are very complex, interrelated molecular and physiological processes that drive the cell movement. Among them, ion transport and pH regulation play a key role, with carbonic anhydrase IX (CA IX) emerging as one of the important "molecular actors." CA IX is a highly active cell surface enzyme expressed in a broad range of solid tumors in response to hypoxia and explored as a clinically useful biomarker of hypoxia and as a therapeutic target. Its biological role is to protect tumor cells from hypoxia and acidosis in the tumor microenvironment. The study published recently by our group showed that CA IX actively contributes to cell migration and invasion. For the first time, we demonstrated CA IX accumulation in lamellipodia of migrating cells and its direct in situ interaction with bicarbonate transporters. Our findings indicate that tumor cells need CA IX not only as a pro-survival factor in hypoxia and acidosis, but also as a pro-migratory component of the cellular apparatus driving epithelial-mesenchymal transition.

Role of Dlg5/lp-dlg, a membrane-associated guanylate kinase family protein, in epithelial-mesenchymal transition in LLc-PK1 renal epithelial cells

Discs large homolog 5 (Dlg5) is a member of the membrane-associated guanylate kinase adaptor family of proteins, some of which are involved in the regulation of epithelial-to-mesenchymal transition (EMT). Dlg5 has been described as a susceptibility gene for Crohn's disease; however, the physiological function of Dlg5 is unknown. We show here that transforming growth factor-β (TGF-β)-induced EMT suppresses Dlg5 expression in LLc-PK1 cells. Depletion of Dlg5 expression by knockdown promoted the expression of the mesenchymal marker proteins, fibronectin and α-smooth muscle actin, and suppressed the expression of E-cadherin. In addition, activation of JNK and p38, which are stimulated by TGF-β, was enhanced by Dlg5 depletion. Furthermore, inhibition of the TGF-β receptor suppressed the effects of Dlg5 depletion. These observations suggest that Dlg5 is involved in the regulation of TGF-βreceptor-dependent signals and EMT.

Cross-talk between calcium and protein kinase A in the regulation of cell migration

Calcium (Ca(2+)) and the cAMP-dependent protein kinase (PKA) are pleiotropic cellular regulators and both exert powerful, diverse effects on cytoskeletal dynamics, cell adhesion, and cell migration. Localization, by A-kinase-anchoring proteins (AKAPs), of PKA activity to the protrusive leading edge, integrins, and other regulators of cytoskeletal dynamics has emerged as an important facet of its role in cell migration. Additional recent work has firmly established the importance of Ca(2+) influx through mechanosensitive transient receptor potential (TRP) channels and through store-operated Ca(2+) entry (SOCE) in cell migration. Finally, there is considerable evidence showing that these mechanisms of Ca(2+) influx and PKA regulation intersect--and often interact--and thus may work in concert to translate complex extracellular cues into the intracellular biochemical anisotropy required for directional cell migration.