Identification of a PDZ protein, PIST, as a binding partner for Rho effector Rhotekin: biochemical and cell-biological characterization of Rhotekin-PIST interaction

Cystic fibrosis transmembrane conductance regulator-associated ligand protects dopaminergic neurons by differentially regulating metabotropic glutamate receptor 5 in the progression of neurotoxin 6-hydroxydopamine-induced Parkinson's disease model

Due to limitations in early diagnosis and treatments of Parkinson's disease (PD), it is necessary to explore the neuropathological changes that occur early in PD progression and to design neuroprotective therapies to prevent or delay the ongoing degeneration process. Metabotropic glutamate receptor 5 (mGlu₅) has shown both diagnostic and therapeutic potential in preclinical studies on PD. Clinical trials using mGlu₅ negative allosteric modulators to treat PD have, however, raised limitations about the neuroprotective role of mGlu₅. It is likely that mGlu₅ has different regulatory roles in different stages of PD. Here, we investigated a protective role of cystic fibrosis transmembrane conductance regulator-associated ligand (CAL) in the progression of PD by differential regulation of mGlu₅ expression and activity to protect against 6-hydroxydopamine (6-OHDA)-induced neurotoxicity. Following treatment with 6-OHDA, mGlu₅ and CAL expressions were elevated in the early stage and reduced in the late stage, both in vitro and in vivo. Activation of mGlu₅ in the early stage by (RS)-2-chloro-5-hydroxyphenylglycine, or blocking mGlu₅ in the late stage by 2-methyl-6-(phenylethynyl) pyridine, increased cell survival and inhibited apoptosis, but these effects were significantly weakened by knockdown of CAL. CAL alleviated 6-OHDA-induced neurotoxicity by regulating mGlu₅-mediated signaling pathways, thereby maintaining the physiological function of mGlu₅ in different disease stages. In PD rat model, CAL deficiency aggravated 6-OHDA toxicity on dopaminergic neurons and increased motor dysfunction because of lack of regulation of mGlu₅ activity. These data reveal a potential mechanism by which CAL specifically regulates the opposite activity of mGlu₅ in progression of PD to protect against neurotoxicity, suggesting that CAL is a favorable endogenous target for the treatment of PD.

PDZ Scaffold Protein CAL Couples with Metabotropic Glutamate Receptor 5 to Protect Against Cell Apoptosis and Is a Potential Target in the Treatment of Parkinson's Disease

Targeting mGluR5 has been an attractive strategy to modulate glutamate excitotoxicity for neuroprotection. Although human clinical trials using mGluR5 negative allosteric modulators (NAMs) have included some disappointments, recent investigations have added several more attractive small molecules to this field, providing a promise that the identification of more additional strategies to modulate mGluR5 activity might be potentially beneficial for the advancement of PD treatment. Here, we determined the role of the interacting partner CAL (cystic fibrosis transmembrane conductance regulator-associated ligand) in mGluR5-mediated protection in vitro and in vivo. In astroglial C6 cells, CAL deficiency blocked (S)-3, 5-dihydroxyphenylglycine (DHPG)-elicited p-AKT and p-ERK1/2, subsequently prevented group I mGluRs-mediated anti-apoptotic protection, which was blocked by receptor antagonist 1-aminoindan-1, 5-dicarboxylic acid (AIDA), and PI3K or MEK inhibitor LY294002 or U0126. In rotenone-treated MN9D cells, both CAL and mGluR5 expressions were decreased in a time- and dose-dependent manner, and the correlation between these 2 proteins was confirmed by lentivirus-delivered CAL overexpression and knockdown. Moreover, CAL coupled with mGluR5 upregulated mGluR5 protein expression by inhibition of ubiquitin-proteasome-dependent degradation to suppress mGluR5-mediated p-JNK and to protect against cell apoptosis. Additionally, CAL also inhibited rotenone-induced glutamate release to modulate mGluR5 activity. Furthermore, in the rotenone-induced rat model of PD, AAV-delivered CAL overexpression attenuated behavioral deficits and dopaminergic neuronal death, while CAL deficiency aggravated rotenone toxicity. On the other hand, the protective effect of the mGluR5 antagonist MPEP was weakened by knocking down CAL. In vivo experiments also confirmed that CAL inhibited ubiquitination-proteasome-dependent degradation to modulate mGluR5 expression and JNK phosphorylation. Our findings show that CAL protects against cell apoptosis via modulating mGluR5 activity, and may be a new molecular target for an effective therapeutic strategy for PD.

Expression and biological function of rhotekin in gastric cancer through regulating p53 pathway

Background/aim

Gastric cancer (GC) is one of a most threatening cancer globally. Rhotekin (RTKN), a Rho effector, has been reported to be upregulated in GC tissues. This study aimed to investigate the underlying regulatory roles of RTKN in the biological behavior of GC.

Methods

Real-time PCR and Western blotting were carried out to detect the mRNA and protein expression, respectively. Cell Counting Kit-8 and xenograft nude mice model were used to evaluate cell proliferation. Flow cytometry analysis was performed to assess cell cycle distribution and cell apoptosis.

Results

RTKN had high expression level in GC compared with normal tissues. RTKN expression strongly associated with tumor size, TNM stage, lymphnode metastasis and the poor prognosis of patients with GC. Downregulation of RTKN significantly repressed GC cell proliferation, but increased cell population in G1/S phase and induced cell apoptosis. Moreover, the RTKN expression level was related to the p53 signaling pathway and histone deacetylase (HDAC) Class I pathway. RTKN knockdown caused a notable increment in the acetylation level of p53 (Lys382), and the expression of p53-target genes (p21, Bax, and PUMA), as well as a reduction in the expression of a potential deacetylase for p53, HDAC1. Notably, downregulation of HDAC1 had similar effects as RTKN knockdown, and RTKN overexpression could hardly abrogate the effects of HDAC1 knockdown on GC cells.

Conclusion

RTKN could work as an oncogene via regulating HDAC1/p53 and may become a promising treatment strategy for GC.

Functions of Rhotekin, an Effector of Rho GTPase, and Its Binding Partners in Mammals

Rhotekin is an effector protein for small GTPase Rho. This protein consists of a Rho binding domain (RBD), a pleckstrin homology (PH) domain, two proline-rich regions and a C-terminal PDZ (PSD-95, Discs-large, and ZO-1)-binding motif. We, and other groups, have identified various binding partners for Rhotekin and carried out biochemical and cell biological characterization. However, the physiological functions of Rhotekin, per se, are as of yet largely unknown. In this review, we summarize known features of Rhotekin and its binding partners in neuronal tissues and cancer cells.

Overexpression of an Arabidopsis thaliana galactinol synthase gene improves drought tolerance in transgenic rice and increased grain yield in the field

Drought stress has often caused significant decreases in crop production which could be associated with global warming. Enhancing drought tolerance without a grain yield penalty has been a great challenge in crop improvement. Here, we report the Arabidopsis thaliana galactinol synthase 2 gene (AtGolS2) was able to confer drought tolerance and increase grain yield in two different rice (Oryza sativa) genotypes under dry field conditions. The developed transgenic lines expressing AtGolS2 under the control of the constitutive maize ubiquitin promoter (Ubi:AtGolS2) also had higher levels of galactinol than the non-transgenic control. The increased grain yield of the transgenic rice under drought conditions was related to a higher number of panicles, grain fertility and biomass. Extensive confined field trials using Ubi:AtGolS2 transgenic lines in Curinga, tropical japonica and NERICA4, interspecific hybrid across two different seasons and environments revealed the verified lines have the proven field drought tolerance of the Ubi:AtGolS2 transgenic rice. The amended drought tolerance was associated with higher relative water content of leaves, higher photosynthesis activity, lesser reduction in plant growth and faster recovering ability. Collectively, our results provide strong evidence that AtGolS2 is a useful biotechnological tool to reduce grain yield losses in rice beyond genetic differences under field drought stress.

Midbody localization of vinexin recruits rhotekin to facilitate cytokinetic abscission

Vinexin is a SH3 domain-containing adaptor protein that has diverse roles in cell adhesion, signal transduction, gene regulation and stress granule assembly. In this study, we found that vinexin localizes at the midbody during cell division and facilitates cytokinesis. Knockdown of vinexin in HeLa cells delayed the mitotic cell cycle progression and increased the time of cell abscission and the failure to resolve the cytoplasmic bridge. Midbody-localized vinexin is essential for recruiting rhotekin to this structure for cytokinesis because overexpression of a vinexin mutant without a rhotekin-binding motif or knockdown of rhotekin also impaired cytokinetic abscission and increased the number of cells arrested at the midbody stage. Aberrant expression of vinexin and rhotekin in various cancers has been implicated to promote metastasis because of their functions in cell adhesion and signaling. Our findings reveal a novel role of vinexin and rhotekin in cytokinetic abscission and provide another perspective of how both molecules may affect oncogenic transformation via this fundamental cell cycle process.

Scaffolding protein GOPC regulates tight junction structure

GOPC (FIG/PIST/CAL) is a PDZ-domain scaffolding protein that regulates the trafficking of a wide array of proteins, including small GTPases, receptors and cell surface molecules such as cadherin 23 and cystic fibrosis transmembrane regulator. In Madin-Darby canine kidney (MDCK) cells, we find that GOPC localizes to the trans-Golgi network (TGN) but not to the cis- or trans-Golgi cisternae. Colocalization occurs with the early endosome Rab GTPase Rab5 and a TGN/endosome marker Rab14 but not with Rab11, a marker of recycling endosomes. No localization of GOPC was detected to the lateral membranes or tight junctions. Knockdown of GOPC in MDCK cells results in decreased transepithelial resistance and increased paracellular flux. This might be attributable to the compromised trafficking of tight junction components from the TGN, as GOPC-knockdown cells have decreased lateral labeling of the tight junction protein claudin-1 and decreased protein levels of claudin-2. GOPC might mediate the trafficking of newly synthesized tight junction proteins from the TGN to the cell surface or the recycling of these proteins from specialized endosomal compartments.

SIL1, a causative cochaperone gene of Marinesco-Söjgren syndrome, plays an essential role in establishing the architecture of the developing cerebral cortex

Marinesco-Sjögren syndrome (MSS) is a rare autosomal recessively inherited disorder with mental retardation (MR). Recently, mutations in the SIL1 gene, encoding a co-chaperone which regulates the chaperone HSPA5, were identified as a major cause of MSS. We here examined the pathophysiological significance of SIL1 mutations in abnormal corticogenesis of MSS. SIL1-silencing caused neuronal migration delay during corticogenesis ex vivo. While RNAi-resistant SIL1 rescued the defects, three MSS-causing SIL1 mutants tested did not. These mutants had lower affinities to HSPA5 in vitro, and SIL1-HSPA5 interaction as well as HSPA5 function was found to be crucial for neuronal migration ex vivo. Furthermore time-lapse imaging revealed morphological disorganization associated with abnormal migration of SIL1-deficient neurons. These results suggest that the mutations prevent SIL1 from interacting with and regulating HSPA5, leading to abnormal neuronal morphology and migration. Consistent with this, when SIL1 was silenced in cortical neurons in one hemisphere, axonal growth in the contralateral hemisphere was delayed. Taken together, abnormal neuronal migration and interhemispheric axon development may contribute to MR in MSS.

Ubiquitination and degradation of CFTR by the E3 ubiquitin ligase MARCH2 through its association with adaptor proteins CAL and STX6

Golgi-localized cystic fibrosis transmembrane conductance regulator (CFTR)-associated ligand (CAL) and syntaxin 6 (STX6) regulate the abundance of mature, post-ER CFTR by forming a CAL/STX6/CFTR complex (CAL complex) that promotes CFTR degradation in lysosomes. However, the molecular mechanism underlying this degradation is unknown. Here we investigated the interaction of a Golgi-localized, membrane-associated RING-CH E3 ubiquitin ligase, MARCH2, with the CAL complex and the consequent binding, ubiquitination, and degradation of mature CFTR. We found that MARCH2 not only co-immunoprecipitated and co-localized with CAL and STX6, but its binding to CAL was also enhanced by STX6, suggesting a synergistic interaction. In vivo ubiquitination assays demonstrated the ubiquitination of CFTR by MARCH2, and overexpression of MARCH2, like that of CAL and STX6, led to a dose-dependent degradation of mature CFTR that was blocked by bafilomycin A1 treatment. A catalytically dead MARCH2 RING mutant was unable to promote CFTR degradation. In addition, MARCH2 had no effect on a CFTR mutant lacking the PDZ motif, suggesting that binding to the PDZ domain of CAL is required for MARCH2-mediated degradation of CFTR. Indeed, silencing of endogenous CAL ablated the effect of MARCH2 on CFTR. Consistent with its Golgi localization, MARCH2 had no effect on ER-localized ΔF508-CFTR. Finally, siRNA-mediated silencing of endogenous MARCH2 in the CF epithelial cell line CFBE-CFTR increased the abundance of mature CFTR. Taken together, these data suggest that the recruitment of the E3 ubiquitin ligase MARCH2 to the CAL complex and subsequent ubiquitination of CFTR are responsible for the CAL-mediated lysosomal degradation of mature CFTR.

Dynamic functions of RhoA in tumor cell migration and invasion

RhoA is one of the more extensively studied members of the Rho family of small GTPase where it is most readily recognized for its contributions to actin-myosin contractility and stress fiber formation. Accordingly, RhoA function during cell migration has been relegated to the rear of the cell where it mediates retraction of the trailing edge. However, RhoA can also mediate membrane ruffling, lamellae formation and membrane blebbing, thus suggesting an active role in membrane protrusions at the leading edge. With the advent of fluorescence resonance energy transfer (FRET)-based Rho activity reporters, RhoA has been shown to be active at the leading edge of migrating cells where it precedes Rac and Cdc42 activation. These observations demonstrate a remarkable versatility to RhoA signaling, but how RhoA function can switch between contraction and protrusion has remained an enigma. This review highlights recent advances regarding how the cooperation of Rho effector Rhotekin and S100A4 suppresses stress fiber generation to permit RhoA-mediated lamellae formation.

A HaloTag® method for assessing the retrograde axonal transport of the p75 neurotrophin receptor and other proteins in compartmented cultures of rat sympathetic neurons

We have adapted HaloTag® (HT) technology for use in compartmented cultures of rat sympathetic neurons in order to provide a technique that can be broadly applied to studies of the retrograde transport of molecules that play roles in neurotrophin signaling. Transfected neurons expressing HT protein alone, HT protein fused to the p75 neurotrophin receptor (p75NTR) or HT protein fused to tubulin α-1B were maintained in compartmented cultures in which cell bodies and proximal axons of rat sympathetic neurons reside in proximal compartments and their distal axons extend into distal compartments. HT ligand containing a fluorescent tetramethylrhodamine (TMR) label was applied either in the distal compartments or the proximal compartments, and the transport of labeled proteins was assayed by gel fluorescence imaging and TMR immunoblot. HT protein expressed alone displayed little or no retrograde transport. HT protein fused to either the intracellular C-terminus or the extracellular N-terminus of p75NTR was retrogradely transported. The retrograde transport of p75NTR was augmented when the distal axons were provided with nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) or antibodies to BDNF. The anterograde transport of HT protein fused to the N-terminus of tubulin α-1B was also demonstrated. We conclude that retrograde transport of HT fusion proteins provides a powerful and novel approach in studies of axonal transport.

Identification of new binding partners of the chemosensory signaling protein Gγ13 expressed in taste and olfactory sensory cells

Tastant detection in the oral cavity involves selective receptors localized at the apical extremity of a subset of specialized taste bud cells called taste receptor cells (TRCs). The identification of the genes coding for the taste receptors involved in this process have greatly improved our understanding of the molecular mechanisms underlying detection. However, how these receptors signal in TRCs, and whether the components of the signaling cascades interact with each other or are organized in complexes is mostly unexplored. Here we report on the identification of three new binding partners for the mouse G protein gamma 13 subunit (Gγ13), a component of the bitter taste receptors signaling cascade. For two of these Gγ13 associated proteins, namely GOPC and MPDZ, we describe the expression in taste bud cells for the first time. Furthermore, we demonstrate by means of a yeast two-hybrid interaction assay that the C terminal PDZ binding motif of Gγ13 interacts with selected PDZ domains in these proteins. In the case of the PDZ domain-containing protein zona occludens-1 (ZO-1), a major component of the tight junction defining the boundary between the apical and baso-lateral region of TRCs, we identified the first PDZ domain as the site of strong interaction with Gγ13. This association was further confirmed by co-immunoprecipitation experiments in HEK 293 cells. In addition, we present immunohistological data supporting partial co-localization of GOPC, MPDZ, or ZO-1, and Gγ13 in taste buds cells. Finally, we extend this observation to olfactory sensory neurons (OSNs), another type of chemosensory cells known to express both ZO-1 and Gγ13. Taken together our results implicate these new interaction partners in the sub-cellular distribution of Gγ13 in olfactory and gustatory primary sensory cells.

Brain sex differences and function of the fruitless gene in Drosophila

The fruitless (fru) gene in Drosophila plays a pivotal role in the formation of neural circuits underlying gender-specific behaviors. Specific labeling of fru expressing neurons has revealed a core circuit responsible for male courtship behavior.Females with a small number of masculinized neuronal clusters in their brain can initiate male-type courtship behavior. By examining the correlations between the masculinized neurons and behavioral gender type, a male-specific neuronal cluster,named P1, which coexpresses fru and double sex, was identified as a putative trigger center for male-type courtship behavior. P1 neurons extend dendrite to the lateral horn,where multimodal sensory inputs converge. Molecular studies suggest that fru determines the level of masculinization of neurons by orchestrating the transcription of a set of downstream genes, which remain to be identified.

Protein kinase D regulates RhoA activity via rhotekin phosphorylation

The members of the protein kinase D (PKD) family of serine/threonine kinases are major targets for tumor-promoting phorbol esters, G protein-coupled receptors, and activated protein kinase C isoforms (PKCs). The expanding list of cellular processes in which PKDs exert their function via phosphorylation of various substrates include proliferation, apoptosis, migration, angiogenesis, and vesicle trafficking. Therefore, identification of novel PKD substrates is necessary to understand the profound role of this kinase family in signal transduction. Here, we show that rhotekin, an effector of RhoA GTPase, is a novel substrate of PKD. We identified Ser-435 in rhotekin as the potential site targeted by PKD in vivo. Expression of a phosphomimetic S435E rhotekin mutant resulted in an increase of endogenous active RhoA GTPase levels. Phosphorylation of rhotekin by PKD2 modulates the anchoring of the RhoA in the plasma membrane. Consequently, the S435E rhotekin mutant displayed enhanced stress fiber formation when expressed in serum-starved fibroblasts. Our data thus identify a novel role of PKD as a regulator of RhoA activity and actin stress fiber formation through phosphorylation of rhotekin.

RTKN2 Induces NF-KappaB Dependent Resistance to Intrinsic Apoptosis in HEK Cells and Regulates BCL-2 Genes in Human CD4(+) Lymphocytes

The gene for Rhotekin 2 (RTKN2) was originally identified in a promyelocytic cell line resistant to oxysterol-induced apoptosis. It is differentially expressed in freshly isolated CD4⁺ T-cells compared with other hematopoietic cells and is down-regulated following activation of the T-cell receptor. However, very little is known about the function of RTKN2 other than its homology to Rho-GTPase effector, rhotekin, and the possibility that they may have similar roles. Here we show that stable expression of RTKN2 in HEK cells enhanced survival in response to intrinsic apoptotic agents; 25-hydroxy cholesterol and camptothecin, but not the extrinsic agent, TNFα. Inhibitors of NF-KappaB, but not MAPK, reversed the resistance and mitochondrial pro-apoptotic genes, Bax and Bim, were down regulated. In these cells, there was no evidence of RTKN2 binding to the GTPases, RhoA or Rac2. Consistent with the role of RTKN2 in HEK over-expressing cells, suppression of RTKN2 in primary human CD4⁺ T-cells reduced viability and increased sensitivity to 25-OHC. The expression of the pro-apoptotic genes, Bax and Bim were increased while BCL-2 was decreased. In both cell models RTKN2 played a role in the process of intrinsic apoptosis and this was dependent on either NF-KappaB signaling or expression of downstream BCL-2 genes. As RTKN2 is a highly expressed in CD4⁺ T-cells it may play a role as a key signaling switch for regulation of genes involved in T-cell survival.

Interaction of a multi-domain adaptor protein, vinexin, with a Rho-effector, Rhotekin

Among various effector proteins for the Rho small GTPase, the function(s) of Rhotekin is almost unknown. We have identified a multi-domain adaptor protein, vinexin, as a binding partner for Rhotekin, using yeast two-hybrid screening of a human heart library. Rhotekin was found to associate with vinexin in vitro, in COS7 cells, and in brain tissues. The C-terminal Pro-rich motif of Rhotekin exhibited binding to the third SH3 domain of vinexin. The binding was little affected by Rho but was inhibited by activated Cdc42 in COS7 cells. Immunofluorescence analyses revealed partial colocalization of vinexin-alpha with Rhotekin at focal adhesions in REF52 fibroblast cells. These results suggest that Rhotekin forms a complex with vinexin and may play a role at focal adhesions.

Analysis of metagene portraits reveals distinct transitions during kidney organogenesis

Organogenesis is a multistage process, but it has been difficult, by conventional analysis, to separate stages and identify points of transition in developmentally complex organs or define genetic pathways that regulate pattern formation. We performed a detailed time-series examination of global gene expression during kidney development and then represented the resulting data as self-organizing maps (SOMs), which reduced more than 30,000 genes to 650 metagenes. Further clustering of these maps identified potential stages of development and suggested points of stability and transition during kidney organogenesis that are not obvious from either standard morphological analyses or conventional microarray clustering algorithms. We also performed entropy calculations of SOMs generated for each day of development and found correlations with morphometric parameters and expression of candidate genes that may help in orchestrating the transitions between stages of kidney development, as well as macro- and micropatterning of the organ.

The Frizzled family of unconventional G-protein-coupled receptors

The Frizzled (FZD) family of receptors is critically involved in embryogenesis, and there is substantial evidence that members of this family also regulate tissue homeostasis in many different organs in the adult. FZD receptors have seven transmembrane-spanning domains and are activated by the WNT family of lipoglycoproteins. Many aspects of FZD signal transduction and pharmacology are still unclear. In this review, we summarize recent advances and some of the key questions about the molecular pharmacology of FZDs, FZD-associated proteins and signal transduction. We also discuss what little is known about the pharmacological binding profiles and the degree of selectivity of WNTs and other extracellular ligands for FZDs. Finally, we focus on signaling events that occur as a direct consequence of FZD activation, signaling via the central phosphoprotein Dishevelled (DVL) and FZD coupling to heterotrimeric G proteins. Here, we outline the current state of knowledge on FZDs and FZD signal transduction and pinpoint aspects of debate and future directions.

Identification of a cell polarity-related protein, Lin-7B, as a binding partner for a Rho effector, Rhotekin, and their possible interaction in neurons

Rhotekin, an effector of Rho, is highly expressed in the brain but its function(s) in neurons is almost unknown. In an attempt to define the properties of Rhotekin in neuronal cells, we focused on its interaction with polarity-related molecules. In the present study, we identified a PDZ protein, Lin-7B, as a binding partner for Rhotekin by yeast two-hybrid screening of human brain cDNA library. We then found that Rhotekin interacts with Lin-7B in in vitro pull-down assays, and forms an immunocomplex in COS7 cells and the rat brain. The C-terminal three amino acids of Rhotekin were essential for the interaction with Lin-7B. Their binding affinity became increased in the presence of active RhoA in the COS7 cell expression system. In addition, immunohistochemical analyses demonstrated that Lin-7 as well as Rhotekin is enriched in neurons. These results suggest that Lin-7 plays some role in neuronal functions in concert with Rho/Rhotekin signals.