Rap2 function requires palmitoylation and recycling endosome localization

Protein lipidation in the tumor microenvironment: enzymology, signaling pathways, and therapeutics

Protein lipidation is a pivotal post-translational modification that increases protein hydrophobicity and influences their function, localization, and interaction network. Emerging evidence has shown significant roles of lipidation in the tumor microenvironment (TME). However, a comprehensive review of this topic is lacking. In this review, we present an integrated and in-depth literature review of protein lipidation in the context of the TME. Specifically, we focus on three major lipidation modifications: S-prenylation, S-palmitoylation, and N-myristoylation. We emphasize how these modifications affect oncogenic signaling pathways and the complex interplay between tumor cells and the surrounding stromal and immune cells. Furthermore, we explore the therapeutic potential of targeting lipidation mechanisms in cancer treatment and discuss prospects for developing novel anticancer strategies that disrupt lipidation-dependent signaling pathways. By bridging protein lipidation with the dynamics of the TME, our review provides novel insights into the complex relationship between them that drives tumor initiation and progression.

Tao and Rap2l ensure proper Misshapen activation and levels during Drosophila border cell migration

Collective cell migration is fundamental in development, wound healing, and metastasis. During Drosophila oogenesis, border cells (BCs) migrate collectively inside the egg chamber, controlled by the Ste20-like kinase Misshapen (Msn). Msn coordinates the restriction of protrusion formation and contractile forces within the cluster. Here, we demonstrate that Tao acts as an upstream activator of Msn in BCs. Depleting Tao significantly impedes BC migration, producing a phenotype similar to Msn loss of function. Furthermore, we show that the localization of Msn relies on its citron homology (CNH) domain, which interacts with the small GTPase Rap2l. Rap2l promotes the trafficking of Msn to the endolysosomal pathway. Depleting Rap2l elevates Msn levels by reducing its trafficking into late endosomes and increases overall contractility. These data suggest that Tao promotes Msn activation, while global Msn protein levels are controlled via Rap2l and the endolysosomal degradation pathway. Thus, two mechanisms ensure appropriate Msn levels and activation in BCs.

TNIK: A redox sensor in endothelial cell permeability

Dysregulation of endothelial barrier integrity can lead to vascular leak and potentially fatal oedema. TNF-α controls endothelial permeability during inflammation and requires the actin organizing Ezrin-Radixin-Moesin (ERM) proteins. We identified TRAF2 and NCK-interacting kinase (TNIK) as a kinase directly phosphorylating and activating ERM, specifically at the plasma membrane of primary human endothelial cells. TNIK mediates TNF-α-dependent cellular stiffness and paracellular gap formation in vitro and is essential in driving inflammatory oedema formation in vivo. Unlike its homologs, TNIK activity is negatively and reversibly regulated by H2O2-mediated oxidation of C202 within the kinase domain. TNIK oxidation results in intermolecular disulfide bond formation and loss of kinase activity. Pharmacologic inhibition of endogenous reactive oxygen species production in endothelial cells elevated TNIK-dependent ERM phosphorylation, endothelial cell contraction, and cell rounding. Together, we highlight an interplay between TNIK, ERM phosphorylation, and redox signalling in regulating TNF-induced endothelial cell permeability.

Complex interplay between RAS GTPases and RASSF effectors regulates subcellular localization of YAP

RAS GTPases bind effectors to convert upstream cues to changes in cellular function. Effectors of classical H/K/NRAS are defined by RBD/RA domains which recognize the GTP-bound conformation of these GTPases, yet the specificity of RBD/RAs for over 160 RAS superfamily proteins remains poorly explored. We have systematically mapped interactions between BRAF and four RASSF effectors, the largest family of RA-containing proteins, with all RAS, RHO and ARF small GTPases. 39 validated complexes reveal plasticity in RASSF binding, while BRAF demonstrates tight specificity for classical H/K/NRAS. Complex between RASSF5 and diverse RAS GTPases at the plasma membrane can activate Hippo signalling and sequester YAP in the cytosol. RASSF8 undergoes liquid-liquid phase separation and resides in YAP-associated membraneless condensates, which also engage several RAS and RHO GTPases. The poorly studied RASSF3 has been identified as a first potential effector of mitochondrial MIRO proteins, and its co-expression with these GTPases impacts mitochondria and peroxisome distribution. These data reveal the complex nature of GTPase-effector interactions and show their systematic elucidation can reveal completely novel and biologically relevant cellular processes.

TNIK in disease: from molecular insights to therapeutic prospects

TRAF2 and NCK interacting kinase (TNIK), a critical interacting protein kinase, is currently receiving wide attention. TNIK is found in various human body organs and tissues and participates in cell motility, proliferation, and differentiation. On the one hand, its aberrant expression is related to the onset and progression of numerous malignant tumors. On the other hand, TNIK is important in neuronal growth, proliferation, differentiation, and synaptic formation. Thus, the novel therapeutic strategies for targeting TNIK offer a promising direction for cancer, neurological or psychotic disorders. Here, we briefly summarized the biological information of TNIK, reviewed the role and regulatory mechanism in cancer and neuropsychiatric diseases, and introduced the research progress of inhibitors targeting TNIK. Taken together, this review hopes to contribute to the in-depth understanding of the function and regulatory mechanism of TNIK, which is of great significance for revealing the role of TNIK in the occurrence and treatment of diseases.

Dual regulation of Misshapen by Tao and Rap2l promotes collective cell migration

Collective cell migration occurs in various biological processes such as development, wound healing and metastasis. During Drosophila oogenesis, border cells (BC) form a cluster that migrates collectively inside the egg chamber. The Ste20-like kinase Misshapen (Msn) is a key regulator of BC migration coordinating the restriction of protrusion formation and contractile forces within the cluster. Here, we demonstrate that the kinase Tao acts as an upstream activator of Msn in BCs. Depletion of Tao significantly impedes BC migration and produces a phenotype similar to Msn loss-of-function. Furthermore, we show that the localization of Msn relies on its CNH domain, which interacts with the small GTPase Rap2l. Our findings indicate that Rap2l promotes the trafficking of Msn to the endolysosomal pathway. When Rap2l is depleted, the levels of Msn increase in the cytoplasm and at cell-cell junctions between BCs. Overall, our data suggest that Rap2l ensures that the levels of Msn are higher at the periphery of the cluster through the targeting of Msn to the degradative pathway. Together, we identified two distinct regulatory mechanisms that ensure the appropriate distribution and activation of Msn in BCs.

ArhGEF12 activates Rap1A and not RhoA in human dermal microvascular endothelial cells to reduce tumor necrosis factor-induced leak

Overwhelming inflammation in the setting of acute critical illness induces capillary leak resulting in hypovolemia, edema, tissue dysoxia, organ failure and even death. The tight junction (TJ)-dependent capillary barrier is regulated by small GTPases, but the specific regulatory molecules most active in this vascular segment under such circumstances are not well described. We set out to identify GTPase regulatory molecules specific to endothelial cells (EC) that form TJs. Transcriptional profiling of confluent monolayers of TJ-forming human dermal microvascular ECs (HDMECs) and adherens junction only forming-human umbilical vein EC (HUVECs) demonstrate ARHGEF12 is basally expressed at higher levels and is only downregulated in HDMECs by junction-disrupting tumor necrosis factor (TNF). HDMECs depleted of ArhGEF12 by siRNA demonstrate a significantly exacerbated TNF-induced decrease in trans-endothelial electrical resistance and disruption of TJ continuous staining. ArhGEF12 is established as a RhoA-GEF in HUVECs and its knock down would be expected to reduce RhoA activity and barrier disruption. Pulldown of active GEFs from HDMECs depleted of ArhGEF12 and treated with TNF show decreased GTP-bound Rap1A after four hours but increased GTP-bound RhoA after 12 h. In cell-free assays, ArhGEF12 immunoprecipitated from HDMECs is able to activate both Rap1A and RhoA, but not act on Rap2A-C, RhoB-C, or even Rap1B which shares 95% sequence identity with Rap1A. We conclude that in TJ-forming HDMECs, ArhGEF12 selectively activates Rap1A to limit capillary barrier disruption in a mechanism independent of cAMP-mediated Epac1 activation.

Traf2 and NCK Interacting Kinase Is a Critical Regulator of Procollagen I Trafficking and Hepatic Fibrogenesis in Mice

Hepatic fibrosis is driven by deposition of matrix proteins following liver injury. Hepatic stellate cells (HSCs) drive fibrogenesis, producing matrix proteins, including procollagen I, which matures into collagen I following secretion. Disrupting intracellular procollagen processing and trafficking causes endoplasmic reticulum stress and stress-induced HSC apoptosis and thus is an attractive antifibrotic strategy. We designed an immunofluorescence-based small interfering RNA (siRNA) screen to identify procollagen I trafficking regulators, hypothesizing that these proteins could serve as antifibrotic targets. A targeted siRNA screen was performed using immunofluorescence to detect changes in intracellular procollagen I. Tumor necrosis factor receptor associated factor 2 and noncatalytic region of tyrosine kinase-interacting kinase (TNIK) was identified and interrogated in vitro and in vivo using the TNIK kinase inhibitor NCB-0846 or RNA interference-mediated knockdown. Our siRNA screen identified nine genes whose knockdown promoted procollagen I retention, including the serine/threonine kinase TNIK. Genetic deletion or pharmacologic inhibition of TNIK through the small molecule inhibitor NCB-0846 disrupted procollagen I trafficking and secretion without impacting procollagen I expression. To investigate the role of TNIK in liver fibrogenesis, we analyzed human and murine livers, finding elevated TNIK expression in human cirrhotic livers and increased TNIK expression and kinase activity in both fibrotic mouse livers and activated primary human HSCs. Finally, we tested whether inhibition of TNIK kinase activity could limit fibrogenesis in vivo. Mice receiving NCB-0846 displayed reduced CCl₄ -induced fibrogenesis compared to CCl₄ alone, although α-smooth muscle actin levels were unaltered. Conclusions: Our siRNA screen effectively identified TNIK as a key kinase involved in procollagen I trafficking in vitro and hepatic fibrogenesis in vivo.

Ras Family of Small GTPases in CRC: New Perspectives for Overcoming Drug Resistance

Colorectal cancer remains among the cancers with the highest incidence, prevalence, and mortality worldwide. Although the development of targeted therapies against the EGFR and VEGFR membrane receptors has considerably improved survival in these patients, the appearance of resistance means that their success is still limited. Overactivation of several members of the Ras-GTPase family is one of the main actors in both tumour progression and the lack of response to cytotoxic and targeted therapies. This fact has led many resources to be devoted over the last decades to the development of targeted therapies against these proteins. However, they have not been as successful as expected in their move to the clinic so far. In this review, we will analyse the role of these Ras-GTPases in the emergence and development of colorectal cancer and their relationship with resistance to targeted therapies, as well as the status and new advances in the design of targeted therapies against these proteins and their possible clinical implications.

GPCR-mediated YAP/TAZ inactivation in fibroblasts via EPAC1/2, RAP2C, and MAP4K7

Yes-associated protein (YAP) and PDZ-binding motif (TAZ) have emerged as important regulators of pathologic fibroblast activation in fibrotic diseases. Agonism of Gαs-coupled G protein coupled receptors (GPCRs) provides an attractive approach to inhibit the nuclear localization and function of YAP and TAZ in fibroblasts that inhibits or reverses their pathological activation. Agonism of the dopamine D1 GPCR has proven effective in preclinical models of lung and liver fibrosis. However, the molecular mechanisms coupling GPCR agonism to YAP and TAZ inactivation in fibroblasts remain incompletely understood. Here, using human lung fibroblasts, we identify critical roles for the cAMP effectors EPAC1/2, the small GTPase RAP2c, and the serine/threonine kinase MAP4K7 as the essential elements in the downstream signaling cascade linking GPCR agonism to LATS1/2-mediated YAP and TAZ phosphorylation and nuclear exclusion in fibroblasts. We further show that this EPAC/RAP2c/MAP4K7 signaling cascade is essential to the effects of dopamine D1 receptor agonism on reducing fibroblast proliferation, contraction, and extracellular matrix production. Targeted modulation of this cascade in fibroblasts may prove a useful strategy to regulate YAP and TAZ signaling and fibroblast activities central to tissue repair and fibrosis.

The RIT1 C-terminus associates with lipid bilayers via charge complementarity

RIT1 is a member of the Ras superfamily of small GTPases involved in regulation of cellular signaling. Mutations to RIT1 are involved in cancer and developmental disorders. Like many Ras subfamily members, RIT1 is localized to the plasma membrane. However, RIT1 lacks the C-terminal prenylation that helps many other subfamily members adhere to cellular membranes. We used molecular dynamics simulations to examine the mechanisms by which the C-terminal peptide (CTP) of RIT1 associates with lipid bilayers. We show that the CTP is unstructured and that its membrane interactions depend on lipid composition. While a 12-residue region of the CTP binds strongly to anionic bilayers containing phosphatidylserine lipids, the CTP termini fray from the membrane allowing for accommodation of the RIT1 globular domain at the membrane-water interface.

Identification and Analysis of the Blood lncRNA Signature for Liver Cirrhosis and Hepatocellular Carcinoma

As one of the most common malignant tumors, hepatocellular carcinoma (HCC) is the fifth major cause of cancer-associated mortality worldwide. In 90% of cases, HCC develops in the context of liver cirrhosis and chronic hepatitis B virus (HBV) infection is an important etiology for cirrhosis and HCC, accounting for 53% of all HCC cases. To understand the underlying mechanisms of the dynamic chain reactions from normal to HBV infection, from HBV infection to liver cirrhosis, from liver cirrhosis to HCC, we analyzed the blood lncRNA expression profiles from 38 healthy control samples, 45 chronic hepatitis B patients, 46 liver cirrhosis patients, and 46 HCC patients. Advanced machine-learning methods including Monte Carlo feature selection, incremental feature selection (IFS), and support vector machine (SVM) were applied to discover the signature associated with HCC progression and construct the prediction model. One hundred seventy-one key HCC progression-associated lncRNAs were identified and their overall accuracy was 0.823 as evaluated with leave-one-out cross validation (LOOCV). The accuracies of the lncRNA signature for healthy control, chronic hepatitis B, liver cirrhosis, and HCC were 0.895, 0.711, 0.870, and 0.826, respectively. The 171-lncRNA signature is not only useful for early detection and intervention of HCC, but also helpful for understanding the multistage tumorigenic processes of HCC.

Temporal Quantitative Profiling of Newly Synthesized Proteins during Aβ Accumulation

Accumulation of aggregated amyloid beta (Aβ) in the brain is believed to impair multiple cellular pathways and play a central role in Alzheimer's disease pathology. However, how this process is regulated remains unclear. In theory, measuring protein synthesis is the most direct way to evaluate a cell's response to stimuli, but to date, there have been few reliable methods to do this. To identify the protein regulatory network during the development of Aβ deposition in AD, we applied a new proteomic technique to quantitate newly synthesized protein (NSP) changes in the cerebral cortex and hippocampus of 2-, 5-, and 9-month-old APP/PS1 AD transgenic mice. This bio-orthogonal noncanonical amino acid tagging analysis combined PALM (pulse azidohomoalanine labeling in mammals) and HILAQ (heavy isotope labeled AHA quantitation) to reveal a comprehensive dataset of NSPs prior to and post Aβ deposition, including the identification of proteins not previously associated with AD, and demonstrated that the pattern of differentially expressed NSPs is age-dependent. We also found dysregulated vesicle transportation networks including endosomal subunits, coat protein complex I (COPI), and mitochondrial respiratory chain throughout all time points and two brain regions. These results point to a pathological dysregulation of vesicle transportation which occurs prior to Aβ accumulation and the onset of AD symptoms, which may progressively impact the entire protein network and thereby drive neurodegeneration. This study illustrates key pathway regulation responses to the development of AD pathogenesis by directly measuring the changes in protein synthesis and provides unique insights into the mechanisms that underlie AD.

Neuronal Signaling Involved in Neuronal Polarization and Growth: Lipid Rafts and Phosphorylation

Neuronal polarization and growth are developmental processes that occur during neuronal cell differentiation. The molecular signaling mechanisms involved in these events in in vivo mammalian brain remain unclear. Also, cellular events of the neuronal polarization process within a given neuron are thought to be constituted of many independent intracellular signal transduction pathways (the "tug-of-war" model). However, in vivo results suggest that such pathways should be cooperative with one another among a given group of neurons in a region of the brain. Lipid rafts, specific membrane domains with low fluidity, are candidates for the hotspots of such intracellular signaling. Among the signals reported to be involved in polarization, a number are thought to be present or translocated to the lipid rafts in response to extracellular signals. As part of our analysis, we discuss how such novel molecular mechanisms are combined for effective regulation of neuronal polarization and growth, focusing on the significance of the lipid rafts, including results based on recently introduced methods.

Social Stress-Induced Postsynaptic Hyporesponsiveness in Glutamatergic Synapses Is Mediated by PSD-Zip70-Rap2 Pathway and Relates to Anxiety-Like Behaviors

PSD-Zip70 is a postsynaptic protein that regulates glutamatergic synapse formation and maturation by modulation of Rap2 activity. PSD-Zip70 knockout (PSD-Zip70KO) mice exhibit defective glutamatergic synaptic transmission in the prefrontal cortex (PFC) with aberrant Rap2 activation. As prefrontal dysfunction is implicated in the pathophysiology of stress-induced psychiatric diseases, we examined PSD-Zip70KO mice in a social defeat (SD) stress-induced mouse model of depression to investigate stress-induced alterations in synaptic function. Compared with wild-type (WT) mice, PSD-Zip70KO mice exhibited almost normal responses to SD stress in depression-related behaviors such as social activity, anhedonia, and depressive behavior. However, PSD-Zip70KO mice showed enhanced anxiety-like behavior irrespective of stress conditions. The density and size of dendritic spines of pyramidal neurons were reduced in the medial PFC (mPFC) in mice exposed to SD stress. Phosphorylation levels of the AMPA–type glutamate receptor (AMPA-R) GluA2 subunit at Ser880 were prominently elevated in mice exposed to SD stress, indicating internalization of surface-expressed AMPA-Rs and decreased postsynaptic responsiveness. Structural and functional impairments in postsynaptic responsiveness were associated with Rap2 GTPase activation in response to SD stress. Social stress-induced Rap2 activation was regulated by a PSD-Zip70-dependent pathway via interaction with SPAR/PDZ-GEF1. Notably, features such as Rap2 activation, dendritic spine shrinkage, and increased GluA2 phosphorylation were observed in the mPFC of PSD-Zip70KO mice even without SD stress. Together with our previous results, the present findings suggest that SD stress-induced postsynaptic hyporesponsiveness in glutamatergic synapses is mediated by PSD-Zip70-Rap2 signaling pathway and closely relates to anxiety-like behaviors.

AMPA receptors and their minions: auxiliary proteins in AMPA receptor trafficking

To correctly transfer information, neuronal networks need to continuously adjust their synaptic strength to extrinsic stimuli. This ability, termed synaptic plasticity, is at the heart of their function and is, thus, tightly regulated. In glutamatergic neurons, synaptic strength is controlled by the number and function of AMPA receptors at the postsynapse, which mediate most of the fast excitatory transmission in the central nervous system. Their trafficking to, at, and from the synapse, is, therefore, a key mechanism underlying synaptic plasticity. Intensive research over the last 20 years has revealed the increasing importance of interacting proteins, which accompany AMPA receptors throughout their lifetime and help to refine the temporal and spatial modulation of their trafficking and function. In this review, we discuss the current knowledge about the roles of key partners in regulating AMPA receptor trafficking and focus especially on the movement between the intracellular, extrasynaptic, and synaptic pools. We examine their involvement not only in basal synaptic function, but also in Hebbian and homeostatic plasticity. Included in our review are well-established AMPA receptor interactants such as GRIP1 and PICK1, the classical auxiliary subunits TARP and CNIH, and the newest additions to AMPA receptor native complexes.

The cAMP effectors, Rap2b and EPAC, are involved in the regulation of the development of the Coxiella burnetii containing vacuole by altering the fusogenic capacity of the vacuole

Cyclic Adenosine 3′,5′-monophosphate (cAMP) is a key second messenger known to directly regulate not only the protein kinase A (PKA) activity but also other important molecules such as the exchange protein activated by cAMP (EPAC), which is as a guanine nucleotide exchange factor (GEF) of the low molecular weight GTPase, Rap2. Coxiella burnetii is a Gram negative bacterium that survives and grows in a large Coxiella replicative vacuole (CRV), which displays lysosomal and autophagic features. In this report, we present evidence that both, EPAC and its downstream effector Rap2b, were recruited to the CRV. The transient over-expression of the Rap2b wt protein, but not its inactive mutant Rap2b ΔAAX, markedly inhibited the development of the large CRV. Additionally, Rap2b wtinhibited the fusion of early Coxiella phagosomes with the fully developed CRV, indicating that homotypic fusion events are altered in the presence of high levels of Rap2b wt. Likewise, the fusion of endosome/lysosomal compartments (heterotypic fusions) with the large CRV was also affected by the over-expression of this GTPase. Interestingly, cell overexpression of Rap2b wt markedly decreased the levels of the v-SNARE, Vamp7, suggesting that this down-regulation impairs the homotypic and heterotypic fusions events of the Coxiella vacuole.

RAP GTPases and platelet integrin signaling

Platelets are highly specialized cells that continuously patrol the vasculature to ensure its integrity (hemostasis). At sites of vascular injury, they are able to respond to trace amounts of agonists and to rapidly transition from an anti-adhesive/patrolling to an adhesive state (integrin inside-out activation) required for hemostatic plug formation. Pathological conditions that disturb the balance in the underlying signaling processes can lead to unwanted platelet activation (thrombosis) or to an increased bleeding risk. The small GTPases of the RAP subfamily, highly expressed in platelets, are critical regulators of cell adhesion, cytoskeleton remodeling, and MAP kinase signaling. Studies by our group and others demonstrate that RAP GTPases, in particular RAP1A and RAP1B, are the key molecular switches that turn on platelet activation/adhesiveness at sites of injury. In this review, we will summarize major findings on the role of RAP GTPases in platelet biology with a focus on the signaling pathways leading to the conversion of integrins to a high-affinity state.

Protein Lipidation: Occurrence, Mechanisms, Biological Functions, and Enabling Technologies

Protein lipidation, including cysteine prenylation, N-terminal glycine myristoylation, cysteine palmitoylation, and serine and lysine fatty acylation, occurs in many proteins in eukaryotic cells and regulates numerous biological pathways, such as membrane trafficking, protein secretion, signal transduction, and apoptosis. We provide a comprehensive review of protein lipidation, including descriptions of proteins known to be modified and the functions of the modifications, the enzymes that control them, and the tools and technologies developed to study them. We also highlight key questions about protein lipidation that remain to be answered, the challenges associated with answering such questions, and possible solutions to overcome these challenges.

Regulation of Rap GTPases in mammalian neurons

Small GTPases are central regulators of many cellular processes. The highly conserved Rap GTPases perform essential functions in the mammalian nervous system during development and in mature neurons. During neocortical development, Rap1 is required to regulate cadherin- and integrin-mediated adhesion. In the adult nervous system Rap1 and Rap2 regulate the maturation and plasticity of dendritic spine and synapses. Although genetic studies have revealed important roles of Rap GTPases in neurons, their regulation by guanine nucleotide exchange factors (GEFs) that activate them and GTPase activating proteins (GAPs) that inactivate them by stimulating their intrinsic GTPase activity is just beginning to be explored in vivo. Here we review how GEFs and GAPs regulate Rap GTPases in the nervous system with a focus on their in vivo function.

Knockdown of Rap2B Inhibits the Proliferation and Invasion in Hepatocellular Carcinoma Cells

Rap2B, a member of the Ras family of small GTP-binding proteins, was found to be highly expressed in various human tumors and plays an important role in the development of tumors. However, the function of Rap2B in hepatocellular carcinoma (HCC) remains unclear. Therefore, in this study, we investigated the biological functions of Rap2B in HCC and the potential underlying mechanisms. Our results indicated that Rap2B was highly expressed in HCC tissues and cell lines. Rap2B silencing obviously inhibited the proliferation, migration, and invasion of HCC cells, as well as attenuated xenografted tumor growth in vivo. Furthermore, Rap2B silencing greatly reduced the expression levels of phosphorylated focal adhesion kinase (p-FAK), matrix metalloproteinase-2 (MMP-2), and MMP-9 in HCC cells. In conclusion, our data suggest that Rap2B silencing inhibits the proliferation and invasion in HCC cells. Thus, Rap2B may have potential as a treatment for HCC.

Structure, functional regulation and signaling properties of Rap2B

The Ras family small guanosine 5'-triphosphate (GTP)-binding protein Rap2B is is a member of the Ras oncogene family and a novel target of p53 that regulates the p53-mediated pro-survival function of cells. The Rap2B protein shares ~90% homology with Rap2A, and its sequence is 70% identical to other members of the Rap family such as RaplA and RaplB. As a result, Rap2B has been theorized to have similar signaling effectors to the GTPase-binding protein Rap, which mediates various biological functions, including the regulation of sterile 20/mitogen-activated proteins. Since its identification in the early 1990s, Rap2B has elicited a considerable interest. Numerous studies indicate that Rap2B exerts specific biological functions, including binding and stimulating phospholipase C-ε and interferon-γ. In addition, downregulation of Rap2B affects the growth of melanoma cells. The present review summarizes the possible effectors and biological functions of Rap2B. Increasing evidence clearly supports the association between Rap2B function and tumor development. Therefore, it is conceivable that anticancer drugs targeting Rap2B may be generated as novel therapies against cancer.

Control of Dendritic Spine Morphological and Functional Plasticity by Small GTPases

Structural plasticity of excitatory synapses is a vital component of neuronal development, synaptic plasticity, and behaviour. Abnormal development or regulation of excitatory synapses has also been strongly implicated in many neurodevelopmental, psychiatric, and neurodegenerative disorders. In the mammalian forebrain, the majority of excitatory synapses are located on dendritic spines, specialized dendritic protrusions that are enriched in actin. Research over recent years has begun to unravel the complexities involved in the regulation of dendritic spine structure. The small GTPase family of proteins have emerged as key regulators of structural plasticity, linking extracellular signals with the modulation of dendritic spines, which potentially underlies their ability to influence cognition. Here we review a number of studies that examine how small GTPases are activated and regulated in neurons and furthermore how they can impact actin dynamics, and thus dendritic spine morphology. Elucidating this signalling process is critical for furthering our understanding of the basic mechanisms by which information is encoded in neural circuits but may also provide insight into novel targets for the development of effective therapies to treat cognitive dysfunction seen in a range of neurological disorders.

PSD-Zip70 Deficiency Causes Prefrontal Hypofunction Associated with Glutamatergic Synapse Maturation Defects by Dysregulation of Rap2 Activity

Dysregulation of synapse formation and plasticity is closely related to the pathophysiology of psychiatric and neurodevelopmental disorders. The prefrontal cortex (PFC) is particularly important for executive functions such as working memory, cognition, and emotional control, which are impaired in the disorders. PSD-Zip70 (Lzts1/FEZ1) is a postsynaptic density (PSD) protein predominantly expressed in the frontal cortex, olfactory bulb, striatum, and hippocampus. Here we found that PSD-Zip70 knock-out (PSD-Zip70KO) mice exhibit working memory and cognitive defects, and enhanced anxiety-like behaviors. These abnormal behaviors are caused by impaired glutamatergic synapse transmission accompanied by tiny-headed immature dendritic spines in the PFC, due to aberrant Rap2 activation, which has roles in synapse formation and plasticity. PSD-Zip70 modulates the Rap2 activity by interacting with SPAR (spine-associated RapGAP) and PDZ-GEF1 (RapGEF) in the postsynapse. Furthermore, suppression of the aberrant Rap2 activation in the PFC rescued the behavioral defects in PSD-Zip70KO mice. Our data demonstrate a critical role for PSD-Zip70 in Rap2-dependent spine synapse development in the PFC and underscore the importance of this regulation in PFC-dependent behaviors.

SIGNIFICANCE STATEMENT

PSD-Zip70 deficiency causes behavioral defects in working memory and cognition, and enhanced anxiety due to prefrontal hypofunction. This study revealed that PSD-Zip70 plays essential roles in glutamatergic synapse maturation via modulation of the Rap2 activity in the PFC. PSD-Zip70 interacts with both SPAR (spine-associated RapGAP) and PDZ-GEF1 (RapGEF) and modulates the Rap2 activity in postsynaptic sites. Our results provide a novel Rap2-specific regulatory mechanism in synaptic maturation involving PSD-Zip70.

p53 target gene Rap2B regulates the cytoskeleton and inhibits cell spreading

PURPOSE

Cell migration requires spatiotemporal integration of signals that target cytoskeletal. Previous studies have indicated that Rho GTPases are crucial regulators of actin dynamics. As homologs of Rho proteins, the role of Rap2B in the regulation of cytoskeleton and its cell signaling pathway remains unknown.

METHODS

The cellular functions of Rap2B were monitored by Western blotting and immunofluorescence staining in order to characterize the protein level and the cell shape.

RESULTS

Here, we show that expression of Rap2B was induced by nocodazole in a p53-dependent manner. However, Rap2B itself is not necessary for p53-dependent cell cycle arrest. We evidenced that over-expression of Rap2B may inhibit cell spreading by disrupting actin dynamics upon nocodazole treatment, but Rap2B (C180A) mutant does not. In contrast, knockdown of Rap2B promoted cell spreading.

CONCLUSIONS

Altogether, these results revealed that Rap2B plays a pivotal role in cytoskeleton reorganization and subsequently inhibits cell spreading, which could be responsible for cancer metastasis.

Palmitoylation pilots ras to recycling endosomes

We recently showed that palmitoylated Ras proteins (H-Ras and N-Ras) localize intracellularly at recycling endosomes (REs) and that REs act as a way-station for Ras proteins as they move along the post-Golgi exocytic pathway to the plasma membrane (PM). Palmitoylation is essential for H-Ras/N-Ras targeting to REs. H-Ras requires two palmitoyl groups for RE targeting. A lack of either or both palmitoyl groups causes H-Ras to be mislocalized to the endoplasmic reticulum (ER), the Golgi apparatus, or the PM. In this commentary, we summarize recent progress about the Ras trafficking cycle between the endomembranes (endosomes/ER/Golgi) and the PM. We further discuss (1) the critical determinants of RE targeting of lipidated proteins and (2) possible Ras-mediated signaling pathways that originate from REs.

Effector recruitment method to study spatially regulated activation of Ras and Rho GTPases

Ras and Rho family GTPases control a wide variety of cellular processes, and the signaling downstream of these GTPases is influenced by their subcellular localization when activated. Since only a minority of total cellular GTPases is active, observation of the total subcellular distribution of GTPases does not reveal where active GTPases are localized. In this chapter, we describe the use of effector recruitment assays to monitor the subcellular localization of active Ras and Rho family GTPases. The recruitment assay relies on preferential binding of downstream effectors to active GTPases versus inactive GTPases. Tagging the GTPase-binding-domain (GBD) of a downstream effector with a fluorescent protein produces a probe that is recruited to compartments where GTPases are active. We describe an example of a recruitment assay using the GBD of PAK1 to monitor Rac1 activity and explain how the assay can be expanded to determine the subcellular localization of activation of other GTPases.

Mechanisms of isoform specific Rap2 signaling during enterocytic brush border formation

Brush border formation during polarity establishment of intestinal epithelial cells is uniquely governed by the Rap2A GTPase, despite expression of the other highly similar Rap2 isoforms (Rap2B and Rap2C). We investigated the mechanisms of this remarkable specificity and found that Rap2C is spatially segregated from Rap2A signaling as it is not enriched at the apical membrane after polarization. In contrast, both Rap2A and Rap2B are similarly located at Rab11 positive apical recycling endosomes and inside the brush border. However, although Rap2B localizes similarly it is not equally activated as Rap2A during brush border formation. We reveal that the C-terminal hypervariable region allows selective activation of Rap2A, yet this selectivity does not originate from the known differential lipid modifications of this region. In conclusion, we demonstrate that Rap2 specificity during brush border formation is determined by two distinct mechanisms involving segregated localization and selective activation.

β1-adrenergic receptor recycles via a membranous organelle, recycling endosome, by binding with sorting nexin27

In cardiomyocytes, β1-adrenergic receptor (β1-AR) plays an important role in regulating cardiac functions. Upon continuous ligand stimulation, β1-AR is internalized and mostly recycled back to the plasma membrane (PM). The recycling endosome (RE) is one of the membranous organelles involved in the protein recycling pathway. To determine whether RE is involved in the internalization of β1-AR upon ligand stimulation, we evaluated the localization of β1-AR after stimulation with a β-agonist, isoproterenol (Iso), in β1-AR-transfected COS-1 cells. After 30 min of Iso treatment and cell surface labeling with the appropriate antibodies, β1-AR was internalized from PM and translocated into the perinuclear region, the same location as the transferrin receptor, an RE marker. We then evaluated whether sorting nexin 27 (SNX27) participated in the β1-AR recycling pathway. When β1-AR and SNX27 were coexpressed, β1-AR coimmunoprecipitated with SNX27. In addition, shRNA-mediated silencing of SNX27 compromised β1-AR recycling and enhanced its delivery into lysosome. Overall, β1-AR on PM was internalized into RE upon Iso stimulation and recycled by RE through binding with SNX27 in COS-1 cells.

Role of the Rap2/TNIK kinase pathway in regulation of LRP6 stability for Wnt signaling

The Wnt/β-catenin signaling pathway plays critical roles in early embryonic development, stem cell biology and human diseases including cancers. Although Rap2, a member of Ras GTPase family, is essential for the Wnt/β-catenin pathway during the body axis specification in Xenopus embryo, the mechanism underlying its regulation of Wnt signaling remains poorly understood. Here, we show that Rap2 is implicated in control of the stability of Wnt receptor, low-density lipoprotein receptor-related protein 6 (LRP6). Knockdown of Rap2 resulted in the proteasome and/or lysosome-dependent degradation of LRP6 both in the presence and absence of Wnt ligand stimulation. In line with this, constitutively active LRP6 lacking its extracellular domain, which is constitutively phosphorylated and resides in intracellular vesicles, was also degraded in the Rap2-silenced cells. In addition, Rap2 and LRP6 associated physically with each other. Furthermore, we found that TRAF2/Nck-interacting kinase (TNIK), a member of the Ste20 protein family, acts as a downstream effector of Rap2 in control of LRP6 stabilization. Consistently, TNIK could rescue the inhibitory effects of Rap2 depletion on Wnt-dependent gene transcription, reporter activation and neural crest induction. Taken together, these results suggest that Rap2 acts via TNIK to regulate the stability of LRP6 receptor for Wnt/β-catenin signaling.

Emerging roles of recycling endosomes

Cells internalize extracellular solutes, ligands and proteins and lipids in the plasma membrane (PM) by endocytosis. The removal of membrane from the PM is counteracted by endosomal recycling pathways that return the endocytosed proteins and lipids back to the PM. Recycling to the PM can occur from early endosomes. However, many cells have a distinct subpopulation of endosomes that have a mildly acidic pH of 6.5 and are involved in the endosomal recycling. These endosomes are dubbed recycling endosomes (REs). In recent years, studies have begun to reveal that function of REs is not limited to the endosomal recycling. In this review, I summarize the nature of membrane trafficking pathways that pass through REs and the cell biological roles of these pathways.

A role for Rap2 in recycling the extended conformation of LFA-1 during T cell migration

T lymphocytes make use of their major integrin LFA-1 to migrate on surfaces that express ICAM-1 such as blood vessels and inflamed tissue sites. How the adhesions are turned over in order to supply traction for this migration has not been extensively investigated. By following the fate of biotinylated membrane LFA-1 on T lymphocytes, we show in this study that LFA-1 internalization and re-exposure on the plasma membrane are linked to migration. Previously we demonstrated the GTPase Rap2 to be a regulator of LFA-1-mediated migration. SiRNA knockdown of this GTPase inhibits both LFA-1 internalization and also its ability to be re-exposed, indicating that Rap2 participates in recycling of LFA-1 and influences its complete endocytosis-exocytosis cycle. Confocal microscopy images reveal that the intracellular distribution of Rap2 overlaps with endosomal recycling vesicles. Although the homologous GTPase Rap1 is also found on intracellular vesicles and associated with LFA-1 activation, these two homologous GTPases do not co-localize. Little is known about the conformation of the LFA-1 that is recycled. We show that the extended form of LFA-1 is internalized and in Rap2 siRNA-treated T lymphocytes the trafficking of this LFA-1 conformation is disrupted resulting in its intracellular accumulation. Thus LFA-1-mediated migration of T lymphocytes requires Rap2-expressing vesicles to recycle the extended form of LFA-1 that we have previously found to control migration at the leading edge.

DHHC5 protein palmitoylates flotillin-2 and is rapidly degraded on induction of neuronal differentiation in cultured cells

Post-translational palmitoylation of intracellular proteins is mediated by protein palmitoyltransferases belonging to the DHHC family, which share a common catalytic Asp-His-His-Cys (DHHC) motif. Several members have been implicated in neuronal development, neurotransmission, and synaptic plasticity. We previously observed that mice homozygous for a hypomorphic allele of the ZDHHC5 gene are impaired in context-dependent learning and memory. To identify potentially relevant protein substrates of DHHC5, we performed a quantitative proteomic analysis of stable isotope-labeled neuronal stem cell cultures from forebrains of normal and DHHC5-GT (gene-trapped) mice using the bioorthogonal palmitate analog 17-octadecynoic acid. We identified ∼300 17-octadecynoic acid-modified and hydroxylamine-sensitive proteins, of which a subset was decreased in abundance in DHHC5-GT cells. Palmitoylation and oligomerization of one of these proteins (flotillin-2) was abolished in DHHC5-GT neuronal stem cells. In COS-1 cells, overexpression of DHHC5 markedly stimulated the palmitoylation of flotillin-2, strongly suggesting a direct enzyme-substrate relationship. Serendipitously, we found that down-regulation of DHHC5 was triggered within minutes following growth factor withdrawal from normal neural stem cells, a maneuver that is used to induce neural differentiation in culture. The effect was reversible for up to 4 h, and degradation was partially prevented by inhibitors of ubiquitin-mediated proteolysis. These findings suggest that protein palmitoylation can be regulated through changes in DHHC PAT levels in response to differentiation signals.

Induction of Smooth Muscle Cell Migration During Arteriogenesis Is Mediated by Rap2

Objective—

Collateral artery growth or arteriogenesis is the primary means of the circulatory system to maintain blood flow in the face of major arterial occlusions. Arteriogenesis depends on activation of fibroblast growth factor (FGF) receptors, but relatively little is known about downstream mediators of FGF signaling.

Methods and Results—

We screened for signaling components that are activated in response to administration of FGF-2 to cultured vascular smooth muscle cells (VSMCs) and detected a significant increase of Rap2 but not of other Ras family members, which corresponded to a strong upregulation of Rap2 and C-Raf in growing collaterals from rabbits with femoral artery occlusion. Small interfering RNAs directed against Rap2 did not affect FGF-2 induced proliferation of VSMC but strongly inhibited their migration. Inhibition of FGF receptor-1 (FGFR1) signaling by infusion of a sulfonic acid polymer or infection with a dominant-negative FGFR1 adenovirus inhibited Rap2 upregulation and collateral vessel growth. Similarly, expression of dominant-negative Rap2 blocked arteriogenesis, whereas constitutive active Rap2 enhanced collateral vessel growth.

Conclusion—

Rap2 is part of the arteriogenic program and acts downstream of the FGFR1 to stimulate VSMC migration. Specific modulation of Rap2 might be an attractive target to manipulate VSMC migration, which plays a role in numerous pathological processes.

MINK and TNIK differentially act on Rap2-mediated signal transduction to regulate neuronal structure and AMPA receptor function

Misshapen/NIKs (Nck-interacting kinases)-related kinase (MINK) and closely related TRAF2/Nck-interacting kinase (TNIK) are proteins that specifically bind to activated Rap2 and are thus hypothesized to relay its downstream signal transduction. Activated Rap2 has been found to stimulate dendritic pruning, reduce synaptic density and cause removal of synaptic AMPA receptors (AMPA-Rs) (Zhu et al., 2005; Fu et al., 2007). Here we report that MINK and TNIK are postsynaptically enriched proteins whose clustering within dendrites is bidirectionally regulated by the activation state of Rap2. Expression of MINK and TNIK in neurons is required for normal dendritic arborization and surface expression of AMPA receptors. Overexpression of a truncated MINK mutant unable to interact with Rap2 leads to reduced dendritic branching and this MINK-mediated effect on neuronal morphology is dependent upon Rap2 activation. While similarly truncated TNIK also reduces neuronal complexity, its effect does not require Rap2 activity. Furthermore, Rap2-mediated removal of surface AMPA-Rs from spines is entirely abrogated by coexpression of MINK, but not TNIK. Thus, although both MINK and TNIK bind GTP-bound Rap2, these kinases employ distinct mechanisms to modulate Rap2-mediated signaling. MINK appears to antagonize Rap2 signal transduction by binding to activated Rap2. We suggest that MINK interaction with Rap2 plays a critical role in maintaining the morphological integrity of dendrites and synaptic transmission.

Structural determinants allowing endolysosomal sorting and degradation of endosomal GTPases

Rapid control of protein degradation is usually achieved through the ubiquitin-proteasome pathway. We recently found that the short-lived GTPase RhoB is degraded in lysosomes. Moreover, the fusion of the RhoB C-terminal sequence CINCCKVL, containing the isoprenylation and palmitoylation sites, to other proteins directs their sorting into multivesicular bodies (MVBs) and rapid lysosomal degradation. Here, we show that this process is highly specific for RhoB. Alteration of late endosome lipid dynamics produced the accumulation of RhoB, but not of other endosomal GTPases, including Rab5, Rab7, Rab9 or Rab11, into enlarged MVB. Other isoprenylated and bipalmitoylated GTPases, such as H-Ras, Rap2A, Rap2B and TC10, were not accumulated into MVB and were stable. Remarkably, although TC10, which is highly homologous to RhoB, was stable, a sequence derived from its C-terminus (CINCCLIT) elicited MVB sorting and degradation of a green fluorescent protein (GFP)-chimeric protein. This led us to identify a cluster of basic amino acids (KKH) in the TC10 hypervariable region, constituting a secondary signal potentially involved in electrostatic interactions with membrane lipids. Mutation of this cluster allowed TC10 MVB sorting and degradation, whereas inserting it into RhoB hypervariable region rescued this protein from its lysosomal degradation pathway. These findings define a highly specific structural module for entering the MVB pathway and rapid lysosomal degradation.