Pragmin, a novel effector of Rnd2 GTPase, stimulates RhoA activity

A comparative analysis of paxillin and Hic-5 proximity interactomes

Focal adhesions serve as structural and signaling hubs, facilitating bidirectional communication at the cell-extracellular matrix interface. Paxillin and the related Hic-5 (TGFβ1i1) are adaptor/scaffold proteins that recruit numerous structural and regulatory proteins to focal adhesions, where they perform both overlapping and discrete functions. In this study, paxillin and Hic-5 were expressed in U2OS osteosarcoma cells as biotin ligase (BioID2) fusion proteins and used as bait proteins for proximity-dependent biotinylation in order to directly compare their respective interactomes. The fusion proteins localized to both focal adhesions and the centrosome, resulting in biotinylation of components of each of these structures. Biotinylated proteins were purified and analyzed by mass spectrometry. The list of proximity interactors for paxillin and Hic-5 comprised numerous shared core focal adhesion proteins that likely contribute to their similar functions in cell adhesion and migration, as well as proteins unique to paxillin and Hic-5 that have been previously localized to focal adhesions, the centrosome, or the nucleus. Western blotting confirmed biotinylation and enrichment of FAK and vinculin, known interactors of Hic-5 and paxillin, as well as several potentially unique proximity interactors of Hic-5 and paxillin, including septin 7 and ponsin, respectively. Further investigation into the functional relationship between the unique interactors and Hic-5 or paxillin may yield novel insights into their distinct roles in cell migration.

Regulation of Cellular-Signaling Pathways by Mammalian Proteins Containing Bacterial EPIYA or EPIYA-Like Motifs Predicted to be Phosphorylated

The effector proteins of several pathogenic bacteria contain the Glu-Pro-Ile-Tyr-Ala (EPIYA) motif or other similar motifs. The EPIYA motif is delivered into the host cells by type III and IV secretion systems, through which its tyrosine residue undergoes phosphorylation by host kinases. These motifs atypically interact with a wide range of Src homology 2 (SH2) domain-containing mammalian proteins through tyrosine phosphorylation, which leads to the perturbation of multiple signaling cascades, the spread of infection, and improved bacterial colonization. Interestingly, it has been reported that EPIYA (or EPIYA-like) motifs exist in mammalian proteomes and regulate mammalian cellular-signaling pathways, leading to homeostasis and disease pathophysiology. It is possible that pathogenic bacteria have exploited EPIYA (or EPIYA-like) motifs from mammalian proteins and that the mammalian EPIYA (or EPIYA-like) motifs have evolved to have highly specific interactions with SH2 domain-containing proteins. In this review, we focus on the regulation of mammalian cellular-signaling pathways by mammalian proteins containing these motifs.

Molecular basis and current insights of atypical Rho small GTPase in cancer

Atypical Rho GTPases are a subtype of the Rho GTPase family that are involved in diverse cellular processes. The typical Rho GTPases, led by RhoA, Rac1 and Cdc42, have been well studied, while relative studies on atypical Rho GTPases are relatively still limited and have great exploration potential. With the increase in studies, current evidence suggests that atypical Rho GTPases regulate multiple biological processes and play important roles in the occurrence and development of human cancers. Therefore, this review mainly discusses the molecular basis of atypical Rho GTPases and their roles in cancer. We summarize the sequence characteristics, subcellular localization and biological functions of each atypical Rho GTPase. Moreover, we review the recent advances and potential mechanisms of atypical Rho GTPases in the development of multiple cancers. A comprehensive understanding and extensive exploration of the biological functions of atypical Rho GTPases and their molecular mechanisms in tumors will provide important insights into the pathophysiology of tumors and the development of cancer therapeutic strategies.

Molecular mechanisms of corpus callosum development: a four-step journey

The Corpus Callosum (CC) is a bundle of axons connecting the cerebral hemispheres. It is the most recent structure to have appeared during evolution of placental mammals. Its development is controlled by a very complex interplay of many molecules. In humans it contains almost 80% of all commissural axons in the brain. The formation of the CC can be divided into four main stages, each controlled by numerous intracellular and extracellular molecular factors. First, a newborn neuron has to specify an axon, leave proliferative compartments, the Ventricular Zone (VZ) and Subventricular Zone (SVZ), migrate through the Intermediate Zone (IZ), and then settle at the Cortical Plate (CP). During the second stage, callosal axons navigate toward the midline within a compact bundle. Next stage is the midline crossing into contralateral hemisphere. The last step is targeting a defined area and synapse formation. This review provides an insight into these four phases of callosal axons development, as well as a description of the main molecular players involved.

Dynamic fusion of genomics and functional network connectivity in UK biobank reveals static and time-varying SNP manifolds

Many psychiatric and neurological disorders show significant heritability, indicating strong genetic influence. In parallel, dynamic functional network connectivity (dFNC) measures functional temporal coupling between brain networks in a time-varying manner and has proven to identify disease-related changes in the brain. However, it remains largely unclear how genetic risk contributes to brain dysconnectivity that further manifests into clinical symptoms. The current work aimed to address this gap by proposing a novel joint ICA (jICA)-based "dynamic fusion" framework to identify dynamically tuned SNP manifolds by linking static SNPs to dynamic functional information of the brain. The sliding window approach was utilized to estimate four dFNC states and compute subject-level state-specific dFNC features. Each state of dFNC features were then combined with 12946 SZ risk SNPs for jICA decomposition, resulting in four parallel fusions in 32861 European ancestry individuals within the UK Biobank cohort. The identified joint SNP-dFNC components were further validated for SZ relevance in an aggregated SZ cohort, and compared for across-state similarity to indicate level of dynamism. The results supported that dynamic fusion yielded "static" and "dynamic" components (i.e., high and low across-state similarity, respectively) for SNP and dFNC modalities. As expected, the SNP components presented a mixture of static and dynamic manifolds, with the latter largely driven by fusion with dFNC. We also showed that some of the dynamic SNP manifolds uniquely elicited by fusion with state-specific dFNC features complemented each other in terms of biological interpretation. This dynamic fusion framework thus allows expanding the SNP modality to manifolds in the time dimension, which provides a unique lens to elicit unique SNP correlates of dFNC otherwise unseen, promising additional insights on how genetic risk links to disease-related dysconnectivity.

Investigating the Protective Effects of a Citrus Flavonoid on the Retardation Morphogenesis of the Oligodendroglia-like Cell Line by Rnd2 Knockdown

Recent discoveries suggest links between abnormalities in cell morphogenesis in the brain and the functional deficiency of molecules controlling signal transduction in glial cells such as oligodendroglia. Rnd2 is one such molecule and one of the Rho family monomeric GTP-binding proteins. Despite the currently known functions of Rnd2, its precise roles as it relates to cell morphogenesis and disease state remain to be elucidated. First, we showed that signaling through the loss of function of the rnd2 gene affected the regulation of oligodendroglial cell-like morphological differentiation using the FBD-102b cell line, which is often utilized as a differentiation model. The knockdown of Rnd2 using the clustered regularly interspaced palindromic repeats (CRISPR)/CasRx system or RNA interference was shown to slow morphological differentiation. Second, the knockdown of Prag1 or Fyn kinase, a signaling molecule acting downstream of Rnd2, slowed differentiation. Rnd2 or Prag1 knockdown also decreased Fyn phosphorylation, which is critical for its activation and for oligodendroglial cell differentiation and myelination. Of note, hesperetin, a citrus flavonoid with protective effects on oligodendroglial cells and neurons, can recover differentiation states induced by the knockdown of Rnd2/Prag1/Fyn. Here, we showed that signaling through Rnd2/Prag1/Fyn is involved in the regulation of oligodendroglial cell-like morphological differentiation. The effects of knocking down the signaling cascade molecule can be recovered by hesperetin, highlighting an important molecular structure involved in morphological differentiation.

Candidate Genes for IgA Nephropathy in Pediatric Patients: Exome-Wide Association Study

IgA nephropathy (IgAN) is an autoimmune disorder which is believed to be non-monogenic. We performed an exome-wide association study of 70 children with IgAN and 637 healthy donors. The HLA allele frequencies were compared between the patients and healthy donors from the bone marrow registry of the Pirogov University. We tested 78,020 gene markers for association and performed functional enrichment analysis and transcription factor binding preference detection. We identified 333 genetic variants, employing three inheritance models. The most significant association with the disorder was observed for rs143409664 (PRAG1) in the case of the additive and dominant models (PBONF = 1.808 × 10-15 and PBONF = 1.654 × 10-15, respectively), and for rs13028230 (UBR3) in the case of the recessive model (PBONF = 1.545 × 10-9). Enrichment analysis indicated the strongly overrepresented "immune system" and "kidney development" terms. The HLA-DQA1*01:01:01G allele (p = 0.0076; OR, 2.021 [95% CI, 1.322-3.048]) was significantly the most frequent among IgAN patients. Here, we characterized, for the first time, the genetic background of Russian IgAN patients, identifying the risk alleles typical of the population. The most important signals were detected in previously undescribed loci.

Structural mapping of PEAK pseudokinase interactions identifies 14-3-3 as a molecular switch for PEAK3 signaling

PEAK pseudokinases regulate cell migration, invasion and proliferation by recruiting key signaling proteins to the cytoskeleton. Despite lacking catalytic activity, alteration in their expression level is associated with several aggressive cancers. Here, we elucidate the molecular details of key PEAK signaling interactions with the adapter proteins CrkII and Grb2 and the scaffold protein 14-3-3. Our findings rationalize why the dimerization of PEAK proteins has a crucial function in signal transduction and provide biophysical and structural data to unravel binding specificity within the PEAK interactome. We identify a conserved high affinity 14-3-3 motif on PEAK3 and demonstrate its role as a molecular switch to regulate CrkII binding and signaling via Grb2. Together, our studies provide a detailed structural snapshot of PEAK interaction networks and further elucidate how PEAK proteins, especially PEAK3, act as dynamic scaffolds that exploit adapter proteins to control signal transduction in cell growth/motility and cancer.

Structural insights into regulation of the PEAK3 pseudokinase scaffold by 14-3-3

PEAK pseudokinases are molecular scaffolds which dimerize to regulate cell migration, morphology, and proliferation, as well as cancer progression. The mechanistic role dimerization plays in PEAK scaffolding remains unclear, as there are no structures of PEAKs in complex with their interactors. Here, we report the cryo-EM structure of dimeric PEAK3 in complex with an endogenous 14-3-3 heterodimer. Our structure reveals an asymmetric binding mode between PEAK3 and 14-3-3 stabilized by one pseudokinase domain and the SHED domain of the PEAK3 dimer. The binding interface contains a canonical phosphosite-dependent primary interaction and a unique secondary interaction not observed in previous structures of 14-3-3/client complexes. Additionally, we show that PKD regulates PEAK3/14-3-3 binding, which when prevented leads to PEAK3 nuclear enrichment and distinct protein-protein interactions. Altogether, our data demonstrate that PEAK3 dimerization forms an unusual secondary interface for 14-3-3 binding, facilitating 14-3-3 regulation of PEAK3 localization and interactome diversity.

Advances in Understanding the Molecular Mechanisms of Neuronal Polarity

The establishment and maintenance of neuronal polarity are important for neural development and function. Abnormal neuronal polarity establishment commonly leads to a variety of neurodevelopmental disorders. Over the past three decades, with the continuous development and improvement of biological research methods and techniques, we have made tremendous progress in the understanding of the molecular mechanisms of neuronal polarity establishment. The activity of positive and negative feedback signals and actin waves are both essential in this process. They drive the directional transport and aggregation of key molecules of neuronal polarity, promote the spatiotemporal regulation of ordered and coordinated interactions of actin filaments and microtubules, stimulate the specialization and growth of axons, and inhibit the formation of multiple axons. In this review, we focus on recent advances in these areas, in particular the important findings about neuronal polarity in two classical models, in vitro primary hippocampal/cortical neurons and in vivo cortical pyramidal neurons, and discuss our current understanding of neuronal polarity.​.

Oncogenic Signalling of PEAK2 Pseudokinase in Colon Cancer

Simple Summary

Catalytically inactive kinases, also named pseudokinases, play important roles in the regulation of cell growth and adhesion. While frequently deregulated in human cancer, their role in tumour development is partially elucidated. Here, we report an important tumour function for the pseudokinase PEAK2 in colorectal cancer (CRC) and propose that PEAK2 upregulation can affect cancer cell adhesive properties through an ABL-dependent mechanism to enable cancer progression. Therefore, targeting PEAK2 oncogenic activity with small tyrosine kinases (TK) inhibitors may be of therapeutic interest in colorectal cancer (CRC).

Abstract

The PEAK family pseudokinases are essential components of tyrosine kinase (TK) pathways that regulate cell growth and adhesion; however, their role in human cancer remains unclear. Here, we report an oncogenic activity of the pseudokinase PEAK2 in colorectal cancer (CRC). Notably, high PRAG1 expression, which encodes PEAK2, was associated with a bad prognosis in CRC patients. Functionally, PEAK2 depletion reduced CRC cell growth and invasion in vitro, while its overexpression increased these transforming effects. PEAK2 depletion also reduced CRC development in nude mice. Mechanistically, PEAK2 expression induced cellular protein tyrosine phosphorylation, despite its catalytic inactivity. Phosphoproteomic analysis identified regulators of cell adhesion and F-actin dynamics as PEAK2 targets. Additionally, PEAK2 was identified as a novel ABL TK activator. In line with this, PEAK2 expression localized at focal adhesions of CRC cells and induced ABL-dependent formation of actin-rich plasma membrane protrusions filopodia that function to drive cell invasion. Interestingly, all these PEAK2 transforming activities were regulated by its main phosphorylation site, Tyr413, which implicates the SRC oncogene. Thus, our results uncover a protumoural function of PEAK2 in CRC and suggest that its deregulation affects adhesive properties of CRC cells to enable cancer progression.

Production and purification of the PEAK pseudokinases for structural and functional studies

The PEAK family of pseudokinases, which comprises PEAK1, PEAK2 and PEAK3, are newly identified scaffolds that dynamically assemble oncogenic signaling pathways known to contribute to the development of several aggressive cancers. A striking feature of this unique family of pseudokinase scaffolds is their large multi-domain structure, which allows them to achieve protein complex assemblies through their structural plasticity and functional versatility. Recent structural advances have begun to reveal the critical regulatory elements that control their function. Specifically, the dimer-dependent scaffolding activity of PEAK pseudokinases is emerging as a critical mechanism for their signaling function, in addition to their ability to hetero-associate to form higher-order regulatory networks to diversify and amplify their signaling output. Here, we present a suite of techniques that enable the efficient expression and purification of PEAK proteins for functional characterization.

SHED-Dependent Oncogenic Signaling of the PEAK3 Pseudo-Kinase

Simple Summary

The human kinome is composed of about 50 pseudo-kinases with unclear function, because they are predicted to be catalytically inactive; however, they are shown to play an important role in cancer, similar to active kinases. Understanding how these pseudo-kinases promote tumor formation despite their catalytic inactivity is a great challenge, which may lead to innovative anti-cancer therapies. The PEAK1 and 2 pseudo-kinases have emerged as important components of the protein tyrosine kinase pathway implicated in cancer progression. They can signal using a scaffolding mechanism via a conserved split helical dimerization (SHED) module. In this study, we uncovered a similar SHED-dependent oncogenic activity for PEAK3, a recently discovered new member of this family. We also show that this new signaling mechanism may be implicated in acute myeloid leukemia.

Abstract

The PEAK1 and Pragmin/PEAK2 pseudo-kinases have emerged as important components of the protein tyrosine kinase pathway implicated in cancer progression. They can signal using a scaffolding mechanism that involves a conserved split helical dimerization (SHED) module. We recently identified PEAK3 as a novel member of this family based on structural homology; however, its signaling mechanism remains unclear. In this study, we found that, although it can self-associate, PEAK3 shows higher evolutionary divergence than PEAK1/2. Moreover, the PEAK3 protein is strongly expressed in human hematopoietic cells and is upregulated in acute myeloid leukemia. Functionally, PEAK3 overexpression in U2OS sarcoma cells enhanced their growth and migratory properties, while its silencing in THP1 leukemic cells reduced these effects. Importantly, an intact SHED module was required for these PEAK3 oncogenic activities. Mechanistically, through a phosphokinase survey, we identified PEAK3 as a novel inducer of AKT signaling, independent of growth-factor stimulation. Then, proteomic analyses revealed that PEAK3 interacts with the signaling proteins GRB2 and ASAP1/2 and the protein kinase PYK2, and that these interactions require the SHED domain. Moreover, PEAK3 activated PYK2, which promoted PEAK3 tyrosine phosphorylation, its association with GRB2 and ASAP1, and AKT signaling. Thus, the PEAK1-3 pseudo-kinases may use a conserved SHED-dependent mechanism to activate specific signaling proteins to promote oncogenesis.

Role of RhoC in cancer cell migration

Migration is one of the five major behaviors of cells. Although RhoC-a classic member of the Rho gene family-was first identified in 1985, functional RhoC data have only been widely reported in recent years. Cell migration involves highly complex signaling mechanisms, in which RhoC plays an essential role. Cell migration regulated by RhoC-of which the most well-known function is its role in cancer metastasis-has been widely reported in breast, gastric, colon, bladder, prostate, lung, pancreatic, liver, and other cancers. Our review describes the role of RhoC in various types of cell migration. The classic two-dimensional cell migration cycle constitutes cell polarization, adhesion regulation, cell contraction and tail retraction, most of which are modulated by RhoC. In the three-dimensional cell migration model, amoeboid migration is the most classic and well-studied model. Here, RhoC modulates the formation of membrane vesicles by regulating myosin II, thereby affecting the rate and persistence of amoeba-like migration. To the best of our knowledge, this review is the first to describe the role of RhoC in all cell migration processes. We believe that understanding the detail of RhoC-regulated migration processes will help us better comprehend the mechanism of cancer metastasis. This will contribute to the study of anti-metastatic treatment approaches, aiding in the identification of new intervention targets for therapeutic or genetic transformational purposes.

Rnd2 differentially regulates oligodendrocyte myelination at different developmental periods

In the CNS, oligodendrocyte precursor cells differentiate into oligodendrocytes to wrap their plasma membranes around neuronal axons, generating mature neural networks with myelin sheaths according to spatial and temporal patterns. While myelination is known to be one of the most dynamic cell morphological changes, the overall intrinsic and extrinsic molecular cues controlling myelination remain to be fully clarified. Here, we describe the biphasic roles of Rnd2, an atypical branch of the Rho family GTPase, in oligodendrocyte myelination during development and after maturation in mice. Compared with littermate controls, oligodendrocyte-specific Rnd2 knockout mice exhibit decreased myelin thickness at the onset of myelination but increased myelin thickness in the later period. Larger proportions of Rho kinase and its substrate Mbs, the signaling unit that negatively regulates oligodendrocyte myelination, are phosphorylated at the onset of myelination, while their smaller proportions are phosphorylated in the later period. In addition, we confirm the biphasic role of Rnd2 through experiments with oligodendrocyte-specific Rnd2 transgenic mice. We conclude that Rnd2 positively regulates myelination in the early myelinating period and negatively regulates myelination in the later period. This unique modulator thus plays different roles depending on the myelination period.

The atypical Rho GTPase Rnd2 is critical for dentate granule neuron development and anxiety-like behavior during adult but not neonatal neurogenesis

Despite the central role of Rho GTPases in neuronal development, their functions in adult hippocampal neurogenesis remain poorly explored. Here, by using a retrovirus-based loss-of-function approach in vivo, we show that the atypical Rho GTPase Rnd2 is crucial for survival, positioning, somatodendritic morphogenesis, and functional maturation of adult-born dentate granule neurons. Interestingly, most of these functions are specific to granule neurons generated during adulthood since the deletion of Rnd2 in neonatally-born granule neurons only affects dendritogenesis. In addition, suppression of Rnd2 in adult-born dentate granule neurons increases anxiety-like behavior whereas its deletion in pups has no such effect, a finding supporting the adult neurogenesis hypothesis of anxiety disorders. Thus, our results are in line with the view that adult neurogenesis is not a simple continuation of earlier processes from development, and establish a causal relationship between Rnd2 expression and anxiety.

Transcriptome Analysis of the Cerebellum of Mice Fed a Manganese-Deficient Diet

Manganese (Mn), primarily acquired through diet, is required for brain function and development. Epidemiological studies have found an association between both low and high levels of Mn and impaired neurodevelopment in children. Recent genetic studies have revealed that patients with congenital Mn deficiency display severe psychomotor disability and cerebral and cerebellar atrophy. Although the impact of Mn on gene expression is beginning to be appreciated, Mn-dependent gene expression remains to be explored in vertebrate animals. The goal of this study was to use a mouse model to define the impact of a low-Mn diet on brain metal levels and gene expression. We interrogated gene expression changes in the Mn-deficient mouse brain at the genome-wide scale by RNA-seq analysis of the cerebellum of mice fed low or normal Mn diets. A total of 137 genes were differentially expressed in Mn-deficient cerebellums compared with Mn-adequate cerebellums (Padj < 0.05). Mn-deficient mice displayed downregulation of key pathways involved with "focal adhesion," "neuroactive ligand-receptor interaction," and "cytokine-cytokine receptor interaction" and upregulation of "herpes simplex virus 1 infection," "spliceosome," and "FoxO signaling pathway." Reactome pathway analysis identified upregulation of the splicing-related pathways and transcription-related pathways, as well as downregulation of "metabolism of carbohydrate," and "extracellular matrix organization," and "fatty acid metabolism" reactomes. The recurrent identifications of splicing-related pathways suggest that Mn deficiency leads to upregulation of splicing machineries and downregulation of diverse biological pathways.

Pathophysiological functions of Rnd proteins

Rnd proteins constitute a subfamily of Rho GTPases represented in mammals by Rnd1, Rnd2 and Rnd3. Despite their GTPase structure, their specific feature is the inability to hydrolyse GTP-bound nucleotide. This aspect makes them atypical among Rho GTPases. Rnds are regulated for their expression at the transcriptional or post-transcriptional levels and they are activated through post-translational modifications and interactions with other proteins. Rnd proteins are mainly involved in the regulation of the actin cytoskeleton and cell proliferation. Whereas Rnd3 is ubiquitously expressed, Rnd1 and 2 are tissue-specific. Increasing data has described their important role during development and diseases. Herein, we describe their involvement in physiological and pathological conditions with a focus on the neuronal and vascular systems, and summarize their implications in tumorigenesis.

The PEAK family of pseudokinases, their role in cell signalling and cancer

The study of pseudokinases has uncovered that catalysis-independent functions play a critical role in cell signalling regulation. However, how pseudokinases dynamically assemble and regulate oncogenic signalling pathways remains, in most cases, unclear due to a limited knowledge of the structural determinants that are critical for their functions. Here, we review the recent progress made to unravel the role of the PEAK family of pseudokinases, which comprises SgK269, SgK223 and the recently identified PEAK3, in assembling specific oncogenic signalling pathways that contribute to the progression of several aggressive cancers. We focus on recent structural advances revealing that SgK269 and SgK223 can homo- and heteroassociate via a unique dimerisation domain, comprising conserved regulatory helices directly surrounding the pseudokinase domain, which is also conserved in PEAK3. We also highlight a potential oligomerisation mechanism driven by the pseudokinase domain. While it is likely that homo- or heterodimerisation and oligomerisation mechanisms contribute to the assembly of complex signalling hubs and provide a means to spatially and temporally modulate and diversify signalling outputs, the exact role that these oncogenic scaffolds play in regulating cell migration, invasion and morphology remains unclear. Here, we attempt to link their structural characteristics to their cellular functions by providing a thorough analysis of the signalling transduction pathways they are known to modulate.

Emerging concepts in pseudoenzyme classification, evolution, and signaling

The 21st century is witnessing an explosive surge in our understanding of pseudoenzyme-driven regulatory mechanisms in biology. Pseudoenzymes are proteins that have sequence homology with enzyme families but that are proven or predicted to lack enzyme activity due to mutations in otherwise conserved catalytic amino acids. The best-studied pseudoenzymes are pseudokinases, although examples from other families are emerging at a rapid rate as experimental approaches catch up with an avalanche of freely available informatics data. Kingdom-wide analysis in prokaryotes, archaea and eukaryotes reveals that between 5 and 10% of proteins that make up enzyme families are pseudoenzymes, with notable expansions and contractions seemingly associated with specific signaling niches. Pseudoenzymes can allosterically activate canonical enzymes, act as scaffolds to control assembly of signaling complexes and their localization, serve as molecular switches, or regulate signaling networks through substrate or enzyme sequestration. Molecular analysis of pseudoenzymes is rapidly advancing knowledge of how they perform noncatalytic functions and is enabling the discovery of unexpected, and previously unappreciated, functions of their intensively studied enzyme counterparts. Notably, upon further examination, some pseudoenzymes have previously unknown enzymatic activities that could not have been predicted a priori. Pseudoenzymes can be targeted and manipulated by small molecules and therefore represent new therapeutic targets (or anti-targets, where intervention should be avoided) in various diseases. In this review, which brings together broad bioinformatics and cell signaling approaches in the field, we highlight a selection of findings relevant to a contemporary understanding of pseudoenzyme-based biology.

PEAK3/C19orf35 pseudokinase, a new NFK3 kinase family member, inhibits CrkII through dimerization

Members of the New Kinase Family 3 (NKF3), PEAK1/SgK269 and Pragmin/SgK223 pseudokinases, have emerged as important regulators of cell motility and cancer progression. Here, we demonstrate that C19orf35 (PEAK3), a newly identified member of the NKF3 family, is a kinase-like protein evolutionarily conserved across mammals and birds and a regulator of cell motility. In contrast to its family members, which promote cell elongation when overexpressed in cells, PEAK3 overexpression does not have an elongating effect on cell shape but instead is associated with loss of actin filaments. Through an unbiased search for PEAK3 binding partners, we identified several regulators of cell motility, including the adaptor protein CrkII. We show that by binding to CrkII, PEAK3 prevents the formation of CrkII-dependent membrane ruffling. This function of PEAK3 is reliant upon its dimerization, which is mediated through a split helical dimerization domain conserved among all NKF3 family members. Disruption of the conserved DFG motif in the PEAK3 pseudokinase domain also interferes with its ability to dimerize and subsequently bind CrkII, suggesting that the conformation of the pseudokinase domain might play an important role in PEAK3 signaling. Hence, our data identify PEAK3 as an NKF3 family member with a unique role in cell motility driven by dimerization of its pseudokinase domain.

Dimerization of the Pragmin Pseudo-Kinase Regulates Protein Tyrosine Phosphorylation

The pseudo-kinase and signaling protein Pragmin has been linked to cancer by regulating protein tyrosine phosphorylation via unknown mechanisms. Here we present the crystal structure of the Pragmin 906-1,368 amino acid C terminus, which encompasses its kinase domain. We show that Pragmin contains a classical protein-kinase fold devoid of catalytic activity, despite a conserved catalytic lysine (K997). By proteomics, we discovered that this pseudo-kinase uses the tyrosine kinase CSK to induce protein tyrosine phosphorylation in human cells. Interestingly, the protein-kinase domain is flanked by N- and C-terminal extensions forming an original dimerization domain that regulates Pragmin self-association and stimulates CSK activity. A1329E mutation in the C-terminal extension destabilizes Pragmin dimerization and reduces CSK activation. These results reveal a dimerization mechanism by which a pseudo-kinase can induce protein tyrosine phosphorylation. Further sequence-structure analysis identified an additional member (C19orf35) of the superfamily of dimeric Pragmin/SgK269/PEAK1 pseudo-kinases.

The pseudokinases SgK269 and SgK223: A novel oncogenic alliance in human cancer

Sugen kinases (SgK)269 (also known as PEAK1), and SgK223, an orthologue of rat pragmin and mouse NACK, are human pseudokinases that are implicated in the progression of several cancers. Both are scaffolding proteins that recruit distinct repertoires of signalling proteins and regulate a variety of biological endpoints including cell migration and invasion. To date, SgK269 and SgK223 have been largely studied as separate signalling entities. However, recent work has demonstrated that SgK269 and SgK223 undergo homo- and heterotypic association that determines signal output and biological response. Further characterization of the mechanism of action of these two pseudokinases will provide novel insights into how they promote cancer progression and may reveal novel therapeutic strategies. Here we review their structure, mechanism and function and roles they play in cancer pathogenesis.

Structure of SgK223 pseudokinase reveals novel mechanisms of homotypic and heterotypic association

The mammalian pseudokinase SgK223, and its structurally related homologue SgK269, are oncogenic scaffolds that nucleate the assembly of specific signalling complexes and regulate tyrosine kinase signalling. Both SgK223 and SgK269 form homo- and hetero-oligomers, a mechanism that underpins a diversity of signalling outputs. However, mechanistic insights into SgK223 and SgK269 homo- and heterotypic association are lacking. Here we present the crystal structure of SgK223 pseudokinase domain and its adjacent N- and C-terminal helices. The structure reveals how the N- and C-regulatory helices engage in a novel fold to mediate the assembly of a high-affinity dimer. In addition, we identified regulatory interfaces on the pseudokinase domain required for the self-assembly of large open-ended oligomers. This study highlights the diversity in how the kinase fold mediates non-catalytic functions and provides mechanistic insights into how the assembly of these two oncogenic scaffolds is achieved in order to regulate signalling output.

Pseudokinases lack kinase activity, yet they impact cellular physiology through the regulation of bona fide signaling kinases. Here the authors describe the structure of the SgK223 pseudokinase and its adjacent domains, and identify regulatory interfaces required for self-assembly and downstream signaling.

Upregulation of Rho7 in the temporal lobe tissue of humans with intractable epilepsy

Patients with intractable epilepsy (IE) exhibit an increased risk of premature death, psychosocial dysfunction and decreasing quality of life. The present study aimed to investigate the alteration in the expression of Rho7 in brain tissue from patients with IE, and to examine the association between Rho7 protein expression and IE. Temporal lobe samples were collected from the temporal lobes of 33 patients with IE patients and 10 age‑ and gender‑matched histologically healthy controls. Immunohistochemical staining was conducted to assess the number of Rho7‑positive cells. In addition, double‑label immunofluorescent staining was performed to examine the cellular localization of Rho7. The protein expression of Rho7 was examined using western blotting. Marked immunoreactivity for Rho7 was detected in the IE group, while faint and scattered immunoreactive staining was observed in the control group. The count of Rho7 positive cells in the IE patients was significantly increased compared with the control subjects (23.47±3.9% vs. 12.09±1.05%; P<0.01). Double‑label immunofluorescent staining indicated that Rho7 was primarily expressed in the cell membrane and cytoplasm, and colocalized with neuron‑specific enolase. Western blot analysis demonstrated that the expression of Rho7 in the IE group was significantly increased compared with the control group (0.41±0.031 vs. 0.25±0.025; P<0.01). The results of the present study demonstrated that upregulation of Rho7 immunoreactivity occurs in the brains of patients with IE, suggesting that Rho7 may be associated with the progression of IE or act as a potential treatment target.

The secret life of kinases: insights into non-catalytic signalling functions from pseudokinases

Over the past decade, our understanding of the mechanisms by which pseudokinases, which comprise ∼10% of the human and mouse kinomes, mediate signal transduction has advanced rapidly with increasing structural, biochemical, cellular and genetic studies. Pseudokinases are the catalytically defective counterparts of conventional, active protein kinases and have been attributed functions as protein interaction domains acting variously as allosteric modulators of conventional protein kinases and other enzymes, as regulators of protein trafficking or localisation, as hubs to nucleate assembly of signalling complexes, and as transmembrane effectors of such functions. Here, by categorising mammalian pseudokinases based on their known functions, we illustrate the mechanistic diversity among these proteins, which can be viewed as a window into understanding the non-catalytic functions that can be exerted by conventional protein kinases.

Neurotrophin Promotes Neurite Outgrowth by Inhibiting Rif GTPase Activation Downstream of MAPKs and PI3K Signaling

Members of the well-known semaphorin family of proteins can induce both repulsive and attractive signaling in neural network formation and their cytoskeletal effects are mediated in part by small guanosine 5'-triphosphatase (GTPases). The aim of this study was to investigate the cellular role of Rif GTPase in the neurotrophin-induced neurite outgrowth. By using PC12 cells which are known to cease dividing and begin to show neurite outgrowth responding to nerve growth factor (NGF), we found that semaphorin 6A was as effective as nerve growth factor at stimulating neurite outgrowth in PC12 cells, and that its neurotrophic effect was transmitted through signaling by mitogen-activated protein kinases (MAPKs) and phosphatidylinositol-3-kinase (PI3K). We further found that neurotrophin-induced neurite formation in PC12 cells could be partially mediated by inhibition of Rif GTPase activity downstream of MAPKs and PI3K signaling. In conclusion, we newly identified Rif as a regulator of the cytoskeletal rearrangement mediated by semaphorins.

Structure and function of Helicobacter pylori CagA, the first-identified bacterial protein involved in human cancer

Chronic infection with Helicobacter pylori cagA-positive strains is the strongest risk factor of gastric cancer. The cagA gene-encoded CagA protein is delivered into gastric epithelial cells via bacterial type IV secretion, where it undergoes tyrosine phosphorylation at the Glu-Pro-Ile-Tyr-Ala (EPIYA) motifs. Delivered CagA then acts as a non-physiological scaffold/hub protein by interacting with multiple host signaling molecules, most notably the pro-oncogenic phosphatase SHP2 and the polarity-regulating kinase PAR1/MARK, in both tyrosine phosphorylation-dependent and -independent manners. CagA-mediated manipulation of intracellular signaling promotes neoplastic transformation of gastric epithelial cells. Transgenic expression of CagA in experimental animals has confirmed the oncogenic potential of the bacterial protein. Structural polymorphism of CagA influences its scaffold function, which may underlie the geographic difference in the incidence of gastric cancer. Since CagA is no longer required for the maintenance of established gastric cancer cells, studying the role of CagA during neoplastic transformation will provide an excellent opportunity to understand molecular processes underlying "Hit-and-Run" carcinogenesis.

Homo- and Heterotypic Association Regulates Signaling by the SgK269/PEAK1 and SgK223 Pseudokinases

SgK269/PEAK1 is a pseudokinase and scaffolding protein that plays a critical role in regulating growth factor receptor signal output and is implicated in the progression of several cancers, including those of the breast, colon, and pancreas. SgK269 is structurally related to SgK223, a human pseudokinase that also functions as a scaffold but recruits a distinct repertoire of signaling proteins compared with SgK269. Structural similarities between SgK269 and SgK223 include a predicted α-helical region (designated CH) immediately preceding the conserved C-terminal pseudokinase (PK) domain. Structure-function analyses of SgK269 in MCF-10A mammary epithelial cells demonstrated a critical role for the CH and PK regions in promoting cell migration and Stat3 activation. Characterization of the SgK269 "interactome" by mass spectrometry-based proteomics identified SgK223 as a novel binding partner, and association of SgK269 with SgK223 in cells was dependent on the presence of the CH and PK domains of both pseudokinases. Homotypic association of SgK269 and SgK223 was also demonstrated and exhibited the same structural requirements. Further analysis using pulldowns and size-exclusion chromatography underscored the critical role of the CH region in SgK269/SgK223 association. Importantly, although SgK269 bridged SgK223 to Grb2, it was unable to activate Stat3 or efficiently enhance migration in SgK223 knock-out cells generated by CRISPR/Cas9. These results reveal previously unrecognized interplay between two oncogenic scaffolds and demonstrate a novel signaling mechanism for pseudokinases whereby homotypic and heterotypic association is used to assemble scaffolding complexes with distinct binding properties and hence qualitatively regulate signal output.

Proximity biotinylation provides insight into the molecular composition of focal adhesions at the nanometer scale

Focal adhesions are protein complexes that link metazoan cells to the extracellular matrix through the integrin family of transmembrane proteins. Integrins recruit many proteins to these complexes, referred to as the "adhesome." We used proximity-dependent biotinylation (BioID) in U2OS osteosarcoma cells to label proteins within 15 to 25 nm of paxillin, a cytoplasmic focal adhesion protein, and kindlin-2, which directly binds β integrins. Using mass spectrometry analysis of the biotinylated proteins, we identified 27 known adhesome proteins and 8 previously unknown components close to paxillin. However, only seven of these proteins interacted directly with paxillin, one of which was the adaptor protein Kank2. The proteins in proximity to β integrin included 15 of the adhesion proteins identified in the paxillin BioID data set. BioID also correctly established kindlin-2 as a cell-cell junction protein. By focusing on this smaller data set, new partners for kindlin-2 were found, namely, the endocytosis-promoting proteins liprin β1 and EFR3A, but, contrary to previous reports, not the filamin-binding protein migfilin. A model adhesome based on both data sets suggests that focal adhesions contain fewer components than previously suspected and that paxillin lies away from the plasma membrane. These data not only illustrate the power of using BioID and stable isotope-labeled mass spectrometry to define macromolecular complexes but also enable the correct identification of therapeutic targets within the adhesome.

C-terminal Src kinase-mediated EPIYA phosphorylation of Pragmin creates a feed-forward C-terminal Src kinase activation loop that promotes cell motility

Pragmin is one of the few mammalian proteins containing the Glu‐Pro‐Ile‐Tyr‐Ala (EPIYA) tyrosine‐phosphorylation motif that was originally discovered in the Helicobacter pylori CagA oncoprotein. Following delivery into gastric epithelial cells by type IV secretion and subsequent tyrosine phosphorylation at the EPIYA motifs, CagA serves as an oncogenic scaffold/adaptor that promiscuously interacts with SH2 domain‐containing mammalian proteins such as the Src homology 2 (SH2) domain‐containing protein tyrosine phosphatase‐2 (SHP2) and the C‐terminal Src kinase (Csk), a negative regulator of Src family kinases. Like CagA, Pragmin also forms a physical complex with Csk. In the present study, we found that Pragmin directly binds to Csk by the tyrosine‐phosphorylated EPIYA motif. The complex formation potentiates kinase activity of Csk, which in turn phosphorylates Pragmin on tyrosine‐238 (Y238), Y343, and Y391. As Y391 of Pragmin comprises the EPIYA motif, Pragmin–Csk interaction creates a feed‐forward regulatory loop of Csk activation. Together with the finding that Pragmin and Csk are colocalized to focal adhesions, these observations indicate that the Pragmin–Csk interaction, triggered by Pragmin EPIYA phosphorylation, robustly stimulates the kinase activity of Csk at focal adhesions, which direct cell‐matrix adhesion that regulates cell morphology and cell motility. As a consequence, expression of Pragmin and/or Csk in epithelial cells induces an elongated cell shape with elevated cell scattering in a manner that is mutually dependent on Pragmin and Csk. Deregulation of the Pragmin–Csk axis may therefore induce aberrant cell migration that contributes to tumor invasion and metastasis.

Silencing NACK by siRNA inhibits tumorigenesis in non-small cell lung cancer via targeting Notch1 signaling pathway

Non-small cell lung cancer (NSCLC) is the most common type of lung tumor with poor prognosis, in which the Notch signaling pathway plays an important role. Notch activation complex kinase (NACK) has been reported both as a co-activator and a target gene of the Notch pathway. However, the molecular mechanism of NACK in NSCLC still remains unknown. In this study, the expression of NACK was analyzed in 35 paired NSCLC tumor samples and 2 NSCLC cell lines. MTT assay, cell migration assay, cell invasion assay, flow cytometry assay, and xenograft model were employed to detect the effect of NACK knockdown on the cell proliferation, metastasis, invasion and apoptosis of NSCLC. The relationship between NACK and Notch1 signaling pathway in NSCLC cells was assessed by western blot and luciferase reporter assay. We found that the expression of NACK in the NSCLC tissues and lung cancer cells were significantly increased. High level of NACK expression is remarkable associated with tumor differentiation, lymphatic metastasis, clinical stage and poor survival prognosis. The interference of NACK significantly inhibited cell proliferation, invasion and metastasis through inducing apoptosis in NSCLC cells. The transcriptional activity of related Notch1 target genes were significantly suppressed resulting from NACK knockdown. This study demonstrates that interference of NACK inhibits NSCLC progression through Notch1 signaling pathway and targeting NACK may be an effective approach for NSCLC therapy.

No Major Host Genetic Risk Factor Contributed to A(H1N1)2009 Influenza Severity

While most patients affected by the influenza A(H1N1) pandemic experienced mild symptoms, a small fraction required hospitalization, often without concomitant factors that could explain such a severe course. We hypothesize that host genetic factors could contribute to aggravate the disease. To test this hypothesis, we compared the allele frequencies of 547,296 genome-wide single nucleotide polymorphisms (SNPs) between 49 severe and 107 mild confirmed influenza A cases, as well as against a general population sample of 549 individuals. When comparing severe vs. mild influenza A cases, only one SNP was close to the conventional p = 5×10⁻⁸. This SNP, rs28454025, sits in an intron of the GSK233 gene, which is involved in a neural development, but seems not to have any connections with immunological or inflammatory functions. Indirectly, a previous association reported with CD55 was replicated. Although sample sizes are low, we show that the statistical power in our design was sufficient to detect highly-penetrant, quasi-Mendelian genetic factors. Hence, and assuming that rs28454025 is likely to be a false positive, no major genetic factor was detected that could explain poor influenza A course.

The pseudokinase SgK223 promotes invasion of pancreatic ductal epithelial cells through JAK1/Stat3 signaling

BACKGROUND

Characterization of molecular mechanisms underpinning development of pancreatic ductal adenocarcinoma (PDAC) may lead to the identification of novel therapeutic targets and biomarkers. SgK223, also known as Pragmin, is a pseudokinase and scaffolding protein closely related to SgK269/PEAK1. Both proteins are implicated in oncogenic tyrosine kinase signaling, but their mechanisms and function remain poorly characterized.

METHODS

Expression of SgK223 in PDAC and PDAC cell lines was characterized using gene expression microarrays, mass spectrometry (MS)-based phosphoproteomics and Western blotting. SgK223 was overexpressed in human pancreatic ductal epithelial (HPDE) cells via retroviral transduction, and knocked down in PDAC cells using siRNA. Cell proliferation was determined using a colorimetric cell viability assay, and cell migration and invasion using transwells. Expression of markers of epithelial-mesenchyme transition (EMT) was assayed by quantitative PCR. SgK223 and Stat3 signaling was interrogated by immunoprecipitation, Western blot and gene reporter assays. The functional role of specific kinases and Stat3 was determined using selective small molecule inhibitors.

RESULTS

Elevated site-selective tyrosine phosphorylation of SgK223 was identified in subsets of PDAC cell lines, and increased expression of SgK223 detected in several PDAC cell lines compared to human pancreatic ductal epithelial (HPDE) cells and in PDACs compared to normal pancreas. Expression of SgK223 in HPDE cells at levels comparable to those in PDAC did not alter cell proliferation but led to a more elongated morphology, enhanced migration and invasion and induced gene expression changes characteristic of a partial EMT. While SgK223 overexpression did not affect activation of Erk or Akt, it led to increased Stat3 Tyr705 phosphorylation and Stat3 transcriptional activity, and SgK223 and Stat3 associated in vivo. SgK223-overexpressing cells exhibited increased JAK1 activation, and use of selective inhibitors determined that the increased Stat3 signaling driven by SgK223 was JAK-dependent. Pharmacological inhibition of Stat3 revealed that Stat3 activation was required for the enhanced motility and invasion of SgK223-overexpressing cells.

CONCLUSIONS

Increased expression of SgK223 occurs in PDAC, and overexpression of SgK223 in pancreatic ductal epithelial cells promotes acquisition of a migratory and invasive phenotype through enhanced JAK1/Stat3 signaling. This represents the first association of SgK223 with a particular human cancer, and links SgK223 with a major signaling pathway strongly implicated in PDAC progression.

Function and regulation of Rnd proteins in cortical projection neuron migration

The mammalian cerebral cortex contains a high variety of neuronal subtypes that acquire precise spatial locations and form long or short-range connections to establish functional neuronal circuits. During embryonic development, cortical projection neurons are generated in the areas lining the lateral ventricles and they subsequently undergo radial migration to reach the position of their final maturation within the cortical plate. The control of the neuroblast migratory behavior and the coordination of the migration process with other neurogenic events such as cell cycle exit, differentiation and final maturation are crucial to normal brain development. Among the key regulators of cortical neuron migration, the small GTP binding proteins of the Rho family and the atypical Rnd members play important roles in integrating intracellular signaling pathways into changes in cytoskeletal dynamics and motility behavior. Here we review the role of Rnd proteins during cortical neuronal migration and we discuss both the upstream mechanisms that regulate Rnd protein activity and the downstream molecular pathways that mediate Rnd effects on cell cytoskeleton.

Helicobacter pylori CagA and gastric cancer: a paradigm for hit-and-run carcinogenesis

Helicobacter pylori is a gastric bacterial pathogen that is etiologically linked to human gastric cancer. The cytotoxin-associated gene A (CagA) protein of H. pylori, which is delivered into gastric epithelial cells via bacterial type IV secretion, is an oncoprotein that can induce malignant neoplasms in mammals. Upon delivery, CagA perturbs multiple host signaling pathways by acting as an extrinsic scaffold or hub protein. On one hand, signals aberrantly raised by CagA are integrated into a direct oncogenic insult, whereas on the other hand, they engender genetic instability. Despite its decisive role in the development of gastric cancer, CagA is not required for the maintenance of a neoplastic phenotype in established cancer cells. Therefore, CagA-conducted gastric carcinogenesis progresses through a hit-and-run mechanism in which pro-oncogenic actions of CagA are successively taken over by a series of genetic and/or epigenetic alterations compiled in cancer-predisposing cells during long-standing infection with cagA-positive H. pylori.

NACK is an integral component of the Notch transcriptional activation complex and is critical for development and tumorigenesis

The Notch signaling pathway governs many distinct cellular processes by regulating transcriptional programs. The transcriptional response initiated by Notch is highly cell context dependent, indicating that multiple factors influence Notch target gene selection and activity. However, the mechanism by which Notch drives target gene transcription is not well understood. Herein, we identify and characterize a novel Notch-interacting protein, Notch activation complex kinase (NACK), which acts as a Notch transcriptional coactivator. We show that NACK associates with the Notch transcriptional activation complex on DNA, mediates Notch transcriptional activity, and is required for Notch-mediated tumorigenesis. We demonstrate that Notch1 and NACK are coexpressed during mouse development and that homozygous loss of NACK is embryonic lethal. Finally, we show that NACK is also a Notch target gene, establishing a feed-forward loop. Thus, our data indicate that NACK is a key component of the Notch transcriptional complex and is an essential regulator of Notch-mediated tumorigenesis and development.

Rho GTPases: masters of cell migration

Since their discovery in the late eighties, the role of Rho GTPases in the regulation of cell migration has been extensively studied and has mainly focused on the hallmark family members Rho, Rac, and Cdc42. Recent technological advances in cell biology, such as Rho-family GTPase activity biosensors, studies in 3D, and unbiased RNAi-based screens, have revealed an increasingly complex role for Rho GTPases during cell migration, with many inter-connected functions and a strong dependency on the physical and chemical properties of the surrounding environment. This review aims to give an overview of recent studies on the role of Rho-family GTPase members in the modulation of cell migration in different environments, and discuss future directions.

Temporal regulation of EGF signalling networks by the scaffold protein Shc1

Cell-surface receptors frequently employ scaffold proteins to recruit cytoplasmic targets, but the rationale for this is uncertain. Activated receptor tyrosine kinases, for example, engage scaffolds such as Shc1 that contain phosphotyrosine (pTyr) binding (PTB) domains. Using quantitative mass spectrometry, we find that Shc1 responds to epidermal growth factor (EGF) stimulation through multiple waves of distinct phosphorylation events and protein interactions. Following stimulation, Shc1 rapidly binds a group of proteins that activate pro-mitogenic/survival pathways dependent on recruitment of the Grb2 adaptor to Shc1 pTyr sites. Akt-mediated feedback phosphorylation of Shc1 Ser29 then recruits the Ptpn12 tyrosine phosphatase. This is followed by a sub-network of proteins involved in cytoskeletal reorganization, trafficking and signal termination that binds Shc1 with delayed kinetics, largely through the SgK269 pseudokinase/adaptor protein. Ptpn12 acts as a switch to convert Shc1 from pTyr/Grb2-based signaling to SgK269-mediated pathways that regulate cell invasion and morphogenesis. The Shc1 scaffold therefore directs the temporal flow of signaling information following EGF stimulation.

Rnd3 induces stress fibres in endothelial cells through RhoB

Rnd proteins are atypical Rho family proteins that do not hydrolyse GTP and are instead regulated by expression levels and post-translational modifications. Rnd1 and Rnd3/RhoE induce loss of actin stress fibres and cell rounding in multiple cell types, whereas responses to Rnd2 are more variable. Here we report the responses of endothelial cells to Rnd proteins. Rnd3 induces a very transient decrease in stress fibres but subsequently stimulates a strong increase in stress fibres, in contrast to the reduction observed in other cell types. Rnd2 also increases stress fibres whereas Rnd1 induces a loss of stress fibres and weakening of cell-cell junctions. Rnd3 does not act through any of its known signalling partners and does not need to associate with membranes to increase stress fibres. Instead, it acts by increasing RhoB expression, which is then required for Rnd3-induced stress fibre assembly. Rnd2 also increases RhoB levels. These data indicate that the cytoskeletal response to Rnd3 expression is dependent on cell type and context, and identify regulation of RhoB as a new mechanism for Rnd proteins to affect the actin cytoskeleton.

Rnd1 and Rnd3 targeting to lipid raft is required for p190 RhoGAP activation

The N-terminal region is the lipid raft–targeting determinant. A novel regulatory mechanism is found by which Rnd proteins function as RhoA antagonists, as is a striking mechanism by which differential membrane targeting governs activities of G proteins having similarities in effector interaction.

The Rnd proteins Rnd1, Rnd2, and Rnd3/RhoE are well known as key regulators of the actin cytoskeleton in various cell types, but they comprise a distinct subgroup of the Rho family in that they are GTP bound and constitutively active. Functional differences of the Rnd proteins in RhoA inhibition signaling have been reported in various cell types. Rnd1 and Rnd3 antagonize RhoA signaling by activating p190 RhoGAP, whereas Rnd2 does not. However, all the members of the Rnd family have been reported to bind directly to p190 RhoGAP and equally induce activation of p190 RhoGAP in vitro, and there is no evidence that accounts for the functional difference of the Rnd proteins in RhoA inhibition signaling. Here we report the role of the N-terminal region in signaling. Rnd1 and Rnd3, but not Rnd2, have a KERRA (Lys-Glu-Arg-Arg-Ala) sequence of amino acids in their N-terminus, which functions as the lipid raft-targeting determinant. The sequence mediates the lipid raft targeting of p190 RhoGAP correlated with its activation. Overall, our results demonstrate a novel regulatory mechanism by which differential membrane targeting governs activities of Rnd proteins to function as RhoA antagonists.

Identification of new interacting partners for atypical Rho GTPases: a SILAC-based approach

In contrast to typical Rho GTPases the regulation of atypical Rho GTPases, such as the members of the RhoBTB subfamily, rarely depends on GEFs and/or GAPs. Instead, they are regulated at the level of their expression, by post-translational modifications, by their rate of degradation as well as through binding of diverse cell-specific interactors. Stable Isotope Labeling by Amino acids in Cell culture (SILAC) is a powerful cutting-edge mass-spectrometry-based technology allowing for protein-interaction studies in vitro with removal of false-positive identifications. In this chapter, we describe how the SILAC technology can be applied to the identification of new interacting partners for atypical - constitutively active - Rho GTPases, i.e. RhoBTB3.

COUP-TFI promotes radial migration and proper morphology of callosal projection neurons by repressing Rnd2 expression

During corticogenesis, late-born callosal projection neurons (CPNs) acquire their laminar position through glia-guided radial migration and then undergo final differentiation. However, the mechanisms controlling radial migration and final morphology of CPNs are poorly defined. Here, we show that in COUP-TFI mutant mice CPNs are correctly specified, but are delayed in reaching the cortical plate and have morphological defects during migration. Interestingly, we observed that the rate of neuronal migration to the cortical plate normally follows a low-rostral to high-caudal gradient, similar to that described for COUP-TFI. This gradient is strongly impaired in COUP-TFI(-/-) brains. Moreover, the expression of the Rho-GTPase Rnd2, a modulator of radial migration, is complementary to both these gradients and strongly increases in the absence of COUP-TFI function. We show that COUP-TFI directly represses Rnd2 expression at the post-mitotic level along the rostrocaudal axis of the neocortex. Restoring correct Rnd2 levels in COUP-TFI(-/-) brains cell-autonomously rescues neuron radial migration and morphological transitions. We also observed impairments in axonal elongation and dendritic arborization of COUP-TFI-deficient CPNs, which were rescued by lowering Rnd2 expression levels. Thus, our data demonstrate that COUP-TFI modulates late-born neuron migration and favours proper differentiation of CPNs by finely regulating Rnd2 expression levels.

The role of Rho GTPase proteins in CNS neuronal migration

The architectonics of the mammalian brain arise from a remarkable range of directed cell migrations, which orchestrate the emergence of cortical neuronal layers and pattern brain circuitry. At different stages of cortical histogenesis, specific modes of cell motility are essential to the stepwise formation of cortical architecture. These movements range from interkinetic nuclear movements in the ventricular zone, to migrations of early-born, postmitotic polymorphic cells into the preplate, to the radial migration of precursors of cortical output neurons across the thickening cortical wall, and the vast, tangential migrations of interneurons from the basal forebrain into the emerging cortical layers. In all cases, actomyosin motors act in concert with cell adhesion receptor systems to provide the force and traction needed for forward movement. As key regulators of actin and microtubule cytoskeletons, cell polarity, and adhesion, the Rho GTPases play critical roles in CNS neuronal migration. This review will focus on the different types of migration in the developing neocortex and cerebellar cortex, and the role of the Rho GTPases, their regulators and effectors in these CNS migrations, with particular emphasis on their involvement in radial migration.

Mammalian Pragmin regulates Src family kinases via the Glu-Pro-Ile-Tyr-Ala (EPIYA) motif that is exploited by bacterial effectors

Several pathogenic bacteria have adopted effector proteins that, upon delivery into mammalian cells, undergo tyrosine phosphorylation at the Glu-Pro-Ile-Tyr-Ala (EPIYA) or EPIYA-like sequence motif by host kinases such as Src family kinases (SFKs). This EPIYA phosphorylation triggers complex formation of bacterial effectors with SH2 domain-containing proteins that results in perturbation of host cell signaling and subsequent pathogenesis. Although the presence of such an anomalous protein interaction suggests the existence of a mammalian EPIYA-containing protein whose function is mimicked or subverted by bacterial EPIYA effectors, no molecule that uses the EPIYA motif for biological function has so far been reported in mammals. Here we show that mammalian Pragmin/SgK223 undergoes tyrosine phosphorylation at the EPIYA motif by SFKs and thereby acquires the ability to interact with the SH2 domain of the C-terminal Src kinase (Csk), a negative regulator of SFKs. The Pragmin-Csk interaction prevents translocalization of Csk from the cytoplasm to the membrane and subsequent inactivation of membrane-associated SFKs. As a result, SFK activity is sustained in cells where Pragmin is phosphorylated at the EPIYA motif. Because EPIYA phosphorylation of Pragmin is mediated by SFKs, cytoplasmic sequestration of Csk by Pragmin establishes a positive feedback regulation of SFK activation. Remarkably, the Helicobacter pylori EPIYA effector CagA binds to the Csk SH2 domain in place of Pragmin and enforces membrane recruitment of Csk and subsequent inhibition of SFKs. This work identifies Pragmin as a mammalian EPIYA effector and suggests that bacterial EPIYA effectors target Pragmin to subvert SFKs for successful infection.

The F-BAR protein Rapostlin regulates dendritic spine formation in hippocampal neurons

Pombe Cdc15 homology proteins, characterized by Fer/CIP4 homology Bin-Amphiphysin-Rvs/extended Fer/CIP4 homology (F-BAR/EFC) domains with membrane invaginating property, play critical roles in a variety of membrane reorganization processes. Among them, Rapostlin/formin-binding protein 17 (FBP17) has attracted increasing attention as a critical coordinator of endocytosis. Here we found that Rapostlin was expressed in the developing rat brain, including the hippocampus, in late developmental stages when accelerated dendritic spine formation and maturation occur. In primary cultured rat hippocampal neurons, knockdown of Rapostlin by shRNA or overexpression of Rapostlin-QQ, an F-BAR domain mutant of Rapostlin that has no ability to induce membrane invagination, led to a significant decrease in spine density. Expression of shRNA-resistant wild-type Rapostlin effectively restored spine density in Rapostlin knockdown neurons, whereas expression of Rapostlin deletion mutants lacking the protein kinase C-related kinase homology region 1 (HR1) or Src homology 3 (SH3) domain did not. In addition, knockdown of Rapostlin or overexpression of Rapostlin-QQ reduced the uptake of transferrin in hippocampal neurons. Knockdown of Rnd2, which binds to the HR1 domain of Rapostlin, also reduced spine density and the transferrin uptake. These results suggest that Rapostlin and Rnd2 cooperatively regulate spine density. Indeed, Rnd2 enhanced the Rapostlin-induced tubular membrane invagination. We conclude that the F-BAR protein Rapostlin, whose activity is regulated by Rnd2, plays a key role in spine formation through the regulation of membrane dynamics.

Proneural transcription factors regulate different steps of cortical neuron migration through Rnd-mediated inhibition of RhoA signaling

Little is known of the intracellular machinery that controls the motility of newborn neurons. We have previously shown that the proneural protein Neurog2 promotes the migration of nascent cortical neurons by inducing the expression of the atypical Rho GTPase Rnd2. Here, we show that another proneural factor, Ascl1, promotes neuronal migration in the cortex through direct regulation of a second Rnd family member, Rnd3. Both Rnd2 and Rnd3 promote neuronal migration by inhibiting RhoA signaling, but they control distinct steps of the migratory process, multipolar to bipolar transition in the intermediate zone and locomotion in the cortical plate, respectively. Interestingly, these divergent functions directly result from the distinct subcellular distributions of the two Rnd proteins. Because Rnd proteins also regulate progenitor divisions and neurite outgrowth, we propose that proneural factors, through spatiotemporal regulation of Rnd proteins, integrate the process of neuronal migration with other events in the neurogenic program.

Signaling in migrating neurons: from molecules to networks

During prenatal and postnatal development of the mammalian brain, new neurons are generated by precursor cells that are located in the germinal zones. Subsequently newborn neurons migrate to their destined location in the brain. On the migrational route immature neurons interact via a series of recognition molecules with a plethora of extracellular cues. Stimuli that are conveyed by extracellular cues are translated into complex intracellular signaling networks that eventually enable neuronal migration. In this Focused Review we discuss signaling networks underlying neuronal migration emphasizing molecules and pathways that appear to be neuron-specific.

Diverse functions for the semaphorin receptor PlexinD1 in development and disease

Plexins are a family of single-pass transmembrane proteins that serve as cell surface receptors for Semaphorins during the embryonic development of animals. Semaphorin-Plexin signaling is critical for many cellular aspects of organogenesis, including cell migration, proliferation and survival. Until recently, little was known about the function of PlexinD1, the sole member of the vertebrate-specific PlexinD (PlxnD1) subfamily. Here we review novel findings about PlxnD1's roles in the development of the cardiovascular, nervous and immune systems and salivary gland branching morphogenesis and discuss new insights concerning the molecular mechanisms of PlxnD1 activity.