Protein kinase B phosphorylates AHNAK and regulates its subcellular localization

Tissue-specific RNA methylation prediction from gene expression data using sparse regression models

N6-methyladenosine (m6A) is a highly prevalent and conserved post-transcriptional modification observed in mRNA and long non-coding RNA (lncRNA). Identifying potential m6A sites within RNA sequences is crucial for unraveling the potential influence of the epitranscriptome on biological processes. In this study, we introduce Exp2RM, a novel approach that formulates single-site-based tissue-specific elastic net models for predicting tissue-specific methylation levels utilizing gene expression data. The resulting ensemble model demonstrates robust predictive performance for tissue-specific methylation levels, with an average R-squared value of 0.496 and a median R-squared value of 0.482 across all 22 human tissues. Since methylation distribution varies among tissues, we trained the model to incorporate similar patterns, significantly improves accuracy with the median R-squared value increasing to 0.728. Additonally, functional analysis reveals Exp2RM's ability to capture coefficient genes in relevant biological processes. This study emphasizes the importance of tissue-specific methylation distribution in enhancing prediction accuracy and provides insights into the functional implications of methylation sites.

Proteomic and genetic analyses of influenza A viruses identify pan-viral host targets

Influenza A Virus (IAV) is a recurring respiratory virus with limited availability of antiviral therapies. Understanding host proteins essential for IAV infection can identify targets for alternative host-directed therapies (HDTs). Using affinity purification-mass spectrometry and global phosphoproteomic and protein abundance analyses using three IAV strains (pH1N1, H3N2, H5N1) in three human cell types (A549, NHBE, THP-1), we map 332 IAV-human protein-protein interactions and identify 13 IAV-modulated kinases. Whole exome sequencing of patients who experienced severe influenza reveals several genes, including scaffold protein AHNAK, with predicted loss-of-function variants that are also identified in our proteomic analyses. Of our identified host factors, 54 significantly alter IAV infection upon siRNA knockdown, and two factors, AHNAK and coatomer subunit COPB1, are also essential for productive infection by SARS-CoV-2. Finally, 16 compounds targeting our identified host factors suppress IAV replication, with two targeting CDK2 and FLT3 showing pan-antiviral activity across influenza and coronavirus families. This study provides a comprehensive network model of IAV infection in human cells, identifying functional host targets for pan-viral HDT.

AHNAK2 is a biomarker and a potential therapeutic target of adenocarcinomas

Adenocarcinoma is the second largest histological type of cervical cancer, second only to cervical squamous cell carcinoma. At present, despite the clinical treatment strategies of cervical adenocarcinoma and cervical squamous cell carcinoma being similar, the outcome and prognosis of cervical adenocarcinoma are significantly poor. Therefore, it is urgent to find specific biomarker and therapeutic target for cervical adenocarcinoma. In this study, we aim to reveal and verify the potential biomarkers and therapeutic targets of cervical adenocarcinoma. Weighted correlation network analysis (WGCNA) reveals the differentially-expressed genes significantly related to the histological characteristics of the two cervical cancer subtypes. We select the genes with the top 20 significance for further investigation. Through microarray and immunohistochemical (IHC) analyses of a variety of tumor tissues, we find that among these 20 genes, AHNAK2 is highly expressed not only in cervical adenocarcinoma, but also in multiple of adenocarcinoma tissues, including esophagus, breast and colon, while not in normal gland tissues. In vitro, AHNAK2 knockdown significantly inhibits cell proliferation and migration of adenocarcinoma cell lines. In vivo, AHNAK2 knockdown significantly inhibits tumor progression and metastasis of various adenocarcinomas. RNA-sequencing and bioinformatics analyses suggest that the inhibitory effect of AHNAK2 knockdown on tumor progression is achieved by regulating DNA replication and upregulating Bim expression. Together, we demonstrate that AHNAK2 is a biomarker and a potential therapeutic target for adenocarcinomas.

Multiomics characterization implicates PTK7 in ovarian cancer EMT and cell plasticity and offers strategies for therapeutic intervention

Most patients with ovarian cancer (OC) are diagnosed at a late stage when there are very few therapeutic options and a poor prognosis. This is due to the lack of clearly defined underlying mechanisms or an oncogenic addiction that can be targeted pharmacologically, unlike other types of cancer. Here, we identified protein tyrosine kinase 7 (PTK7) as a potential new therapeutic target in OC following a multiomics approach using genetic and pharmacological interventions. We performed proteomics analyses upon PTK7 knockdown in OC cells and identified novel downstream effectors such as synuclein-γ (SNCG), SALL2, and PP1γ, and these findings were corroborated in ex vivo primary samples using PTK7 monoclonal antibody cofetuzumab. Our phosphoproteomics analyses demonstrated that PTK7 modulates cell adhesion and Rho-GTPase signaling to sustain epithelial-mesenchymal transition (EMT) and cell plasticity, which was confirmed by high-content image analysis of 3D models. Furthermore, using high-throughput drug sensitivity testing (525 drugs) we show that targeting PTK7 exhibited synergistic activity with chemotherapeutic agent paclitaxel, CHK1/2 inhibitor prexasertib, and PLK1 inhibitor GSK461364, among others, in OC cells and ex vivo primary samples. Taken together, our study provides unique insight into the function of PTK7, which helps to define its role in mediating aberrant Wnt signaling in ovarian cancer.

Deleterious AHNAK2 Mutation as a Novel Biomarker for Immune Checkpoint Inhibitors in Non-Small Cell Lung Cancer

Immune checkpoint inhibitors (ICIs) have exhibited promising efficacy in non-small cell lung cancer (NSCLC), but the response occurs in only a minority of patients. In clinic, biomarkers such as TMB (tumor mutation burden) and PD-L1 (programmed cell death 1 ligand 1) still have their limitations in predicting the prognosis of ICI treatment. Hence, reliable predictive markers for ICIs are urgently needed. A public immunotherapy dataset with clinical information and mutational data of 75 NSCLC patients was obtained from cBioPortal as the discovery cohort, and another immunotherapy dataset of 249 patients across multiple cancer types was collected as the validation. Integrated bioinformatics analysis was performed to explore the potential mechanism, and immunohistochemistry studies were used to verify it. AHNAK nucleoprotein 2 (AHNAK2) was reported to have pro-tumor growth effects across multiple cancers, while its role in tumor immunity was unclear. We found that approximately 11% of the NSCLC patients harbored AHNAK2 mutations, which were associated with promising outcomes to ICI treatments (ORR, p = 0.013). We further found that AHNAK2 deleterious mutation (del-AHNAK2^mut) possessed better predictive function in NSCLC than non-deleterious AHNAK2 mutation (PFS, OS, log-rank p < 0.05), potentially associated with stronger tumor immunogenicity and an activated immune microenvironment. This work identified del-AHNAK2^mut as a novel biomarker to predict favorable ICI response in NSCLC.

Interaction between TMEFF1 and AHNAK proteins in ovarian cancer cells: Implications for clinical prognosis

TMEFF1 is a newly discovered protein involved in the physiological functions of the central nervous system, embryonic development, and other biological processes. Our previous study revealed that TMEFF1 acts as a tumor-promoting gene in ovarian cancer. AHNAK, as a giant scaffolding protein, plays a role in the formation of the blood-brain barrier, cell architecture and the regulation of cardiac calcium channels. However, its role in ovarian cancer remains poorly researched. In this study, we detected the expression of AHNAK and TMEFF1 in 148 different ovarian cancer tissues, determined their relationship with pathological parameters and prognosis, clarified the interaction between the two proteins, and explored the related cancer-promoting mechanisms through immunohistochemistry, immunoprecipitation, immunofluorescence double staining, western blotting, and bioinformatics. The high expression of ANHAK and TMEFF1 in ovarian cancer indicated a higher degree of tumor malignancy and a worse prognosis. Furthermore, the expression of TMEFF1 and AHNAK was significantly positively correlated. The results also showed that AHNAK and TMEFF1 co-localized and interacted with each other in ovarian cancer tissues and cells. And knockdown of AHNAK promoted proliferation, migration and invasion of ovarian cancer cells in vitro. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses showed that AHNAK and related genes were enriched during mitosis regulation, cytoskeleton formation, gene epigenetics, etc., whereas TMEFF1 and related genes are enriched during immune regulation and other processes. We also clarified the network of kinases, microRNA, and transcription factor targets, and the impact of genetic mutations on prognosis. Notably, AHNAK was regulated by the expression of TMEFF1 and can activate the MAPK pathways. Overall, high expression of AHNAK and TMEFF1 in ovarian cancer cells indicated a higher degree of tumor malignancy and a worse prognosis. Therefore, the interaction between AHNAK and TMEFF1 may become a potential anti-tumor target for ovarian cancer treatment.

AHNAK: The quiet giant in calcium homeostasis

The phosphoprotein AHNAK is a large, ubiquitously expressed scaffolding protein involved in mediating a host of protein-protein interactions. This enables AHNAK to participate in various multi-protein complexes thereby orchestrating a range of diverse biological processes, including tumour suppression, immune regulation and cell architecture maintenance. A less studied but nonetheless equally important function occurs in calcium homeostasis. It does so by largely interacting with the L-type voltage-gated calcium channel (LVGCC) present in the plasma membrane of excitable cells such as muscles and neurons. Several studies have characterized the underlying basis of AHNAK's functional role in calcium channel modulation, which has led to a greater understanding of this cellular process and its associated pathologies. In this article we review and examine recent advances relating to the physiological aspects of AHNAK in calcium regulation. Specifically, we will provide a broad overview of AHNAK including its structural makeup and its interaction with several isoforms of LVGCC, and how these molecular interactions regulate calcium modulation across various tissues and their implication in muscle and neuronal function.

AHNAK2 Promotes Migration, Invasion, and Epithelial-Mesenchymal Transition in Lung Adenocarcinoma Cells via the TGF-β/Smad3 Pathway

Purpose

Lung adenocarcinoma is one of the common causes of cancer-related deaths worldwide. AHNAKs are giant proteins, which are correlated with cell structure and migration, cardiac calcium channel signaling, and other processes. Current studies identified AHNAK2 as a novel oncogene in some cancers; however, studies on its function in lung cancers are limited.

Materials and Methods

The expression of AHNAK2 was analyzed in normal lung tissues, lung adenocarcinoma tissues, and paracancerous tissues using the Oncomine database. It was further verified in relative cell lines by real-time quantitative polymerase chain reaction and Western blotting (WB). Adenocarcinoma cell lines were transfected with si-NC and si-AHNAK2 by lipofectamine 3000 and treated with or without TGF-β1, and cell migration and invasion were detected by wound-healing and transwell assays. The expression of epithelial-mesenchymal transition (EMT) markers was detected by WB, as well as that of phosphorylated-Smad3 (p-Smad3) and Smad3 levels. After Smad3 phosphorylation inhibitor was added to the adenocarcinoma cell lines, migration and invasion were detected by wound-healing and transwell assays, and the expression of EMT markers was detected by WB when the cells were transfected with si-NC and si-AHNAK2 and treated with or without TGF-β1.

Results

We found higher expression of AHNAK2 in lung adenocarcinoma tissues through the Oncomine database and further verified its high expression in relative cell lines. When the cells were stimulated with TGF-β1, knockdown of AHNAK2 suppressed cell migration, invasion, and EMT, and inhibited TGF-β-induced Smad3 signaling. When p-Smad3 was inhibited, knockdown of AHNAK2 had no effect on the two cell lines investigated when treated with or without TGF-β1.

Conclusion

AHNAK2 acts as an oncogenic protein and promotes migration, invasion, and EMT in lung adenocarcinoma cells via the TGF-β/Smad3 pathway. Thus, it may be a novel target for lung adenocarcinoma therapy.

Functions of Vertebrate Ferlins

Ferlins are multiple-C2-domain proteins involved in Ca²⁺-triggered membrane dynamics within the secretory, endocytic and lysosomal pathways. In bony vertebrates there are six ferlin genes encoding, in humans, dysferlin, otoferlin, myoferlin, Fer1L5 and 6 and the long noncoding RNA Fer1L4. Mutations in DYSF (dysferlin) can cause a range of muscle diseases with various clinical manifestations collectively known as dysferlinopathies, including limb-girdle muscular dystrophy type 2B (LGMD2B) and Miyoshi myopathy. A mutation in MYOF (myoferlin) was linked to a muscular dystrophy accompanied by cardiomyopathy. Mutations in OTOF (otoferlin) can be the cause of nonsyndromic deafness DFNB9. Dysregulated expression of any human ferlin may be associated with development of cancer. This review provides a detailed description of functions of the vertebrate ferlins with a focus on muscle ferlins and discusses the mechanisms leading to disease development.

AHNAK C-Terminal Peptide Membrane Binding-Interactions between the Residues 5654-5673 of AHNAK and Phospholipid Monolayers and Bilayers

The dysferlin membrane repair complex contains a small complex, S100A10-annexin A2, which initiates membrane repair by recruiting the protein AHNAK to the membrane, where it interacts via binding sites in the C-terminal region. However, no molecular data are available for the membrane binding of the various proteins involved in this complex. Therefore, the present study investigated the membrane binding of AHNAK to elucidate its role in the cell membrane repair process. A chemically synthesized peptide (pAHNAK), comprising the 20 amino acids in the C-terminal domain of AHNAK, was applied to Langmuir monolayer models, and the binding parameters and insertion angles were measured with surface tensiometry and ellipsometry. The interaction of pAHNAK with lipid bilayers was studied using ³¹P solid-state nuclear magnetic resonance. pAHNAK preferentially and strongly interacted with phospholipids that comprised negatively charged polar head groups with unsaturated lipids. This finding provides a better understanding of AHNAK membrane behavior and the parameters that influence its function in membrane repair.

1H, 13C and 15N backbone NMR chemical shift assignments of the C-terminal P4 domain of Ahnak

Ahnak is a ~ 700 kDa polypeptide that was originally identified as a tumour-related nuclear phosphoprotein, but later recognized to play a variety of diverse physiological roles related to cell architecture and migration. A critical function of Ahnak is modulation of Ca²⁺ signaling in cardiomyocytes by interacting with the β subunit of the L-type Ca²⁺ channel (Ca_V1.2). Previous studies have identified the C-terminal region of Ahnak, designated as P3 and P4 domains, as a key mediator of its functional activity. We report here the nearly complete ¹H, ¹³C and ¹⁵N backbone NMR chemical shift assignments of the 11 kDa C-terminal P4 domain of Ahnak. This study lays the foundations for future investigations of functional dynamics, structure determination and interaction site mapping of the Ca_V1.2-Ahnak complex.

Fluorescence-Reported Allelic Exchange Mutagenesis Reveals a Role for Chlamydia trachomatis TmeA in Invasion That Is Independent of Host AHNAK

Development of approaches to genetically manipulate Chlamydia is fostering important advances in understanding pathogenesis. Fluorescence-reported allelic exchange mutagenesis (FRAEM) now enables the complete deletion of specific genes in C. trachomatis L2. We have leveraged this technology to delete the coding sequences for a known type III effector. The evidence provided here indicates that CT694/CTL0063 is a virulence protein involved in chlamydial invasion. Based on our findings, we designate the gene product corresponding to ct694-ctl0063translocated membrane-associated effector A (TmeA). Deletion of tmeA did not impact development of intracellular chlamydiae. However, the absence of TmeA manifested as a decrease in infectivity in both tissue culture and murine infection models. The in vitro defect was reflected by impaired invasion of host cells. TmeA binds human AHNAK, and we demonstrate here that AHNAK is transiently recruited by invading chlamydiae. TmeA, however, is not required for endogenous AHNAK recruitment. TmeA also impairs AHNAK-dependent actin bundling activity. This TmeA-mediated effect likely does not explain impaired invasion displayed by the tmeA strain of Chlamydia, since AHNAK-deficient cells revealed no invasion phenotype. Overall, our data indicate the efficacy of FRAEM and reveal a role of TmeA during chlamydial invasion that manifests independently of effects on AHNAK.

Cyclooxygenase 2: protein-protein interactions and posttranslational modifications

Numerous studies implicate the cyclooxygenase 2 (COX2) enzyme and COX2-derived prostanoids in various human diseases, and thus, much effort has been made to uncover the regulatory mechanisms of this enzyme. COX2 has been shown to be regulated at both the transcriptional and posttranscriptional levels, leading to the development of nonsteroidal anti-inflammatory drugs (NSAIDs) and selective COX2 inhibitors (COXIBs), which inhibit the COX2 enzyme through direct targeting. Recently, evidence of posttranslational regulation of COX2 enzymatic activity by s-nitrosylation, glycosylation, and phosphorylation has also been presented. Additionally, posttranslational regulators that actively downregulate COX2 expression by facilitating increased proteasome degradation of this enzyme have also been reported. Moreover, recent data identified proteins, located in close proximity to COX2 enzyme, that serve as posttranslational modulators of COX2 function, upregulating its enzymatic activity. While the precise mechanisms of the protein-protein interaction between COX2 and these regulatory proteins still need to be addressed, it is likely these interactions could regulate COX2 activity either as a result of conformational changes of the enzyme or by impacting subcellular localization of COX2 and thus affecting its interactions with regulatory proteins, which further modulate its activity. It is possible that posttranslational regulation of COX2 enzyme by such proteins could contribute to manifestation of different diseases. The uncovering of posttranslational regulation of COX2 enzyme will promote the development of more efficient therapeutic strategies of indirectly targeting the COX2 enzyme, as well as provide the basis for the generation of novel diagnostic tools as biomarkers of disease.

AHNAK2 is a Novel Prognostic Marker and Oncogenic Protein for Clear Cell Renal Cell Carcinoma

Integrative database analysis was performed to identify novel candidate oncogene AHNAK2 overexpressed in clear cell renal cell carcinoma (ccRCC). However, the function of AHNAK2 in cancer cells is currently unknown. In this study, we first confirmed the upregulation of AHNAK2 in ccRCC tissues compared with adjacent normal tissues in 15 pairs of samples. Then we analyzed AHNAK2 expression in a large cohort of ccRCC patient samples (n = 355), and found that up-regulation of AHNAK2 was positively correlated with tumor progression and poor survival (p = 0.032). Knockdown of AHNAK2 inhibited cancer cell proliferation, colony formation and migration in vitro and tumorigenic ability in vivo. Meanwhile, knockdown of AHNAK2 impaired the cell oncologic-metabolism by inhibiting lipid synthesis. Moreover, we observed that expression of AHNAK2 was greatly upregulated, at least in part, by hypoxia in cancer cells. By using chromatin immune-precipitation (CHIP) and promoter-luciferase reporter assays, we identified that upregulation of AHNAK2 induced by hypoxia was hypoxia-inducible factor-1α (HIF1α)-dependent. Knockdown of AHNAK2 impaired hypoxia-induced epithelial-mesenchymal transition (EMT) and stem cell-like properties. Considered together, we reveal that AHNAK2 is upregulated in cancer cells and hypoxic upregulation of AHNAK2 can drive tumorigenesis and progression by supporting EMT and cancer cell stemness. Thus, AHNAK2 is a novel prognostic marker and an oncogenic protein for ccRCC.

AHNAK: the giant jack of all trades

The nucleoprotein AHNAK is an unusual and somewhat mysterious scaffolding protein characterised by its large size of approximately 700 kDa. Several aspects of this protein remain uncertain, including its exact molecular function and regulation on both the gene and protein levels. Various studies have attempted to annotate AHNAK and, notably, protein interaction and expression analyses have contributed greatly to our current understanding of the protein. The implicated biological processes are, however, very diverse, ranging from a role in the formation of the blood-brain barrier, cell architecture and migration, to the regulation of cardiac calcium channels and muscle membrane repair. In addition, recent evidence suggests that AHNAK might be yet another accomplice in the development of tumour metastasis. This review will discuss the different functional roles of AHNAK, highlighting recent advancements that have added foundation to the proposed roles while identifying ties between them. Implications for related fields of research are noted and suggestions for future research that will assist in unravelling the function of AHNAK are offered.

Nuclear export of L-periaxin, mediated by its nuclear export signal in the PDZ domain

Myelinating Schwann cells specifically express L-periaxin (L-PRX) in the mammalian peripheral nervous system. Several loss-of-function mutations in periaxin have been described and linked to autosomal recessive Dejerine Sottas neuropathy and to demyelinating Charcot-Marie-Tooth disease. The localization of L-periaxin is developmentally regulated in the nucleus and the plasma membrane of Schwann cells. In this study, L-periaxin, which contains a PDZ domain, a nuclear localization signal (NLS) domain, a repeat domain, and an acidic domain, was localized in the cytoplasm of RSC96 cells. By contrast, a mutant L-periaxin with a deleted PDZ domain was localized mainly in the nucleus of RSC96 cells. After a nuclear cyclin A1, which is localized exclusively in the nucleus, was fused with the PDZ domain, cyclinA1was found in the cytoplasm of RSC96 cells. Treatment with leptomycin B (LMB), a specific inhibitor of nuclear export mediated by leucine-rich nuclear export signal (NES), also causes nuclear accumulation of wild-type L-periaxin. Double leucine mutation (L83, 85Q) in the putative NES in the PDZ domain prevented L-periaxin nuclear export and induced nuclear accumulation. These results suggested that the localization of L-periaxin in the cytoplasm is supported by NES in the PDZ domain.

The Annexin A2/p11 complex is required for efficient invasion of Salmonella Typhimurium in epithelial cells

The facultative intracellular pathogen, Salmonella enterica, triggers its own uptake into non-phagocytic epithelial cells. Invasion is dependent on a type 3 secretion system (T3SS), which delivers a cohort of effector proteins across the plasma membrane where they induce dynamic actin-driven ruffling of the membrane and ultimately, internalization of the bacteria into a modified phagosome. In eukaryotic cells, the calcium- and phospholipid-binding protein Annexin A2 (AnxA2) functions as a platform for actin remodelling in the vicinity of dynamic cellular membranes. AnxA2 is mostly found in a stable heterotetramer, with p11, which can interact with other proteins such as the giant phosphoprotein AHNAK. We show here that AnxA2, p11 and AHNAK are required for T3SS-mediated Salmonella invasion of cultured epithelial cells and that the T3SS effector SopB is required for recruitment of AnxA2 and AHNAK to Salmonella invasion sites. Altogether this work shows that, in addition to targeting Rho-family GTPases, Salmonella can intersect the host cell actin pathway via AnxA2.

Determination of phosphoserine/threonine by nano ultra-performance liquid chromatography-tandem mass spectrometry coupled with microscale labeling

Protein phosphorylation is an important regulatory post-translational modification in many biochemical processes. The phosphopeptide analysis strategies developed in this study were all at microscale. After using a standard microwave oven to assist protein digestion, phosphoserine and phosphothreonine were tagged with chemical analogues, such as 2-mercaptoethanol and 3-mercapto-1-propanol, to enable simultaneously relative quantitation and identification. This method enabled the use of thio alcohols for direct labeling of phosphorylated sites (not labeled at the mercapto, amino, hydroxyl, or carboxyl groups) of phosphopeptides. Various digestion parameters (e.g., microwave power, reaction time, NH4HCO3 concentration) and derivatization efficiency parameters (e.g., reaction time, labeling tag concentration) were studied and optimized. In both control and experimental samples, microwave-assisted digestion coupled with relative quantitation using analogue tags enabled calculation of phosphopeptide ratios in the same sequence. A non-labeling method was also established for quantifying phosphopeptides in human plasma by using the abundant protein albumin as an internal control for normalizing relative quantities of phosphopeptides. Nano ultra-performance liquid chromatography (nanoUPLC) was combined with LTQ Orbitrap to enable simultaneous protein relative quantitation and identification. These strategies proved to be effective for quantifying phosphopeptides in biological samples.

Secretome analysis of Glioblastoma cell line--HNGC-2

Glioblastoma multiforme (GBM) is the most common and aggressive type of primary malignant tumor of the central nervous system. We have carried out a deep analysis of the secretome of a rapidly proliferating and tumorigenic cell line HNGC-2, representing GBM, in an effort to identify proteins, which may be targeted in the plasma of GBM patients as markers for diagnosis and disease surveillance. Prefractionation of the proteins from the conditioned medium of HNGC-2 cells in SDS gels followed by LC-MS/MS analysis using an ESI-IT mass spectrometer (LTQ) led to a total of 996 protein identifications with ≥2 peptides each. Of them, 664 proteins were observed in the transcriptome of HNGC-2 cells. The dataset of 996 proteins was mapped to important functional groups, such as cellular assembly and organisation, DNA recombination and repair, and other classes. Actin cytoskeleton signalling, phosphatidyl inositol 3 kinase (PI3K/AKT) and integrin linked kinase (ILK) signalling pathways were seen as enriched pathways. Comparisons with the published secretome of cell lines from 12 different cancers, including GBM, revealed that 348 proteins shared a commonality with a secretome of at least one other cell line, 321 of which were found to contain signal sequences or transmembrane domains and 335 could be linked to a plasma membrane or extracellular localization. Through intergration of this data we arrived at a non-redundant list of 597 protein identifications with the potential for secretion either by classical secretory pathways or by non-secretory processes; 233 of them have been detected in cerebrospinal fluid or plasma as per the published literature, and 172 have been implicated in GBM or other cancers. The HNGC-2 secretome dataset could serve as a useful resource for designing a targeted investigation of GBM biomarkers in plasma.

AHNAK and inflammatory markers predict poor survival in laryngeal carcinoma

AHNAK/Desmoyokin is a giant protein which has been recently linked to reorganization of the actin cytoskeleton, cellular migration and invasion. Here, we investigated the role of AHNAK in the pathophysiology of larynx carcinoma-one of the major subtypes of head and neck cancer. To this end, we analysed AHNAK expression in tumor tissues from 83 larynx carcinoma patients in relation to overall survival. We found that tumoral AHNAK overexpression significantly associated with poor survival of these patients both in univariate and multivariate analysis. In further studies, we combined the prognostic value of AHNAK with selected markers of inflammation, such as macrophage migration inhibitory factor (MIF) and tumor-infiltrating neutrophils (CD66b-positive cells). Both MIF and neutrophils have been linked to enhanced tumoral migration and poor clinical outcome in patients with orohypopharynx carcinoma-another major subtype of head and neck cancer. Interestingly, we found that synchronous high levels of AHNAK and MIF or AHNAK and neutrophils, respectively, were stronger predictors of poor survival than AHNAK alone. Synchronous high levels of all three markers were the strongest predictors of poor survival in our patient cohort. Taken together, our findings propose novel strategies for an accurate prognosis in larynx carcinoma and suggest potential mechanisms of inflammation-mediated tumor progression.

The hinge region of the scaffolding protein of cell contacts, zonula occludens protein 1, regulates interacting with various signaling proteins

Zonula occludens protein 1 (ZO-1) is a ubiquitous scaffolding protein, but it is unknown why it functions in very different cellular contacts. We hypothesized that a specific segment, the unique hinge region, can be bound by very different regulatory proteins. Using surface plasmon resonance spectroscopy and binding assays to peptide libraries, we show, for the first time, that the hinge region directly interacts with disparate signal elements such as G-proteins alpha 12 and alpha i2, the regulator of G-protein signaling 5, multifunctional signaling protein ahnak1, and L-type Ca2+-channel beta-2-subunit. The novel binding proteins specifically bound to a coiled coil-helix predicted in the hinge region of ZO-. The interactions were modulated by phosphorylation in the hinge helix. Activation of the G-proteins influenced their association to ZO-1. In colon cells, G alpha i2 and ZO-1 were associated, as shown by coimmunoprecipitation. After cotransfection in kidney cells, G alpha i2 barely colocalized with ZO-1; the colocalization coefficient was significantly increased when epinephrine activated G-protein signaling. In conclusion, proteins with different regulatory potential adhere to and influence cellular functions of ZO-proteins, and the interactions can be modulated via its hinge region and/or the binding proteins.

Modularity and functional plasticity of scaffold proteins as p(l)acemakers in cell signaling

Cells coordinate and integrate various functional modules that control their dynamics, intracellular trafficking, metabolism and gene expression. Such capacity is mediated by specific scaffold proteins that tether multiple components of signaling pathways at plasma membrane, Golgi apparatus, mitochondria, endoplasmic reticulum, nucleus and in more specialized subcellular structures such as focal adhesions, cell-cell junctions, endosomes, vesicles and synapses. Scaffold proteins act as "pacemakers" as well as "placemakers" that regulate the temporal, spatial and kinetic aspects of protein complex assembly by modulating the local concentrations, proximity, subcellular dispositions and biochemical properties of the target proteins through the intricate use of their modular protein domains. These regulatory mechanisms allow them to gate the specificity, integration and crosstalk of different signaling modules. In addition to acting as physical platforms for protein assembly, many professional scaffold proteins can also directly modify the properties of their targets while they themselves can be regulated by post-translational modifications and/or mechanical forces. Furthermore, multiple scaffold proteins can form alliances of higher-order regulatory networks. Here, we highlight the emerging themes of scaffold proteins by analyzing their common and distinctive mechanisms of action and regulation, which underlie their functional plasticity in cell signaling. Understanding these mechanisms in the context of space, time and force should have ramifications for human physiology and for developing new therapeutic approaches to control pathological states and diseases.

Self-regulated alternative splicing at the AHNAK locus

AHNAK is a 700-kDa protein involved in cytoarchitecture and calcium signaling. It is secondarily reduced in muscle of dysferlinopathy patients and accumulates in muscle of calpainopathy patients, both affected by a muscular dystrophy. AHNAK directly interacts with dysferlin. This interaction is lost on cleavage of AHNAK by the protease calpain 3, explaining the molecular observations in patients. Currently, little is known of AHNAK regulation. We describe the self-regulation of multiple mRNA transcripts emanating from the AHNAK locus in muscle cells. We show that the AHNAK gene consists of a 17-kb exon flanked by multiple small exons. This genetic structure is shared by AHNAK2 and Periaxin, which share a common ancestor. Two major AHNAK transcripts are differentially expressed during muscle differentiation that encode for a small (17-kDa) and a large (700-kDa) protein isoform. These proteins interact in the cytoplasm, but the small AHNAK is also present in the nucleus. During muscle differentiation the small AHNAK is strongly increased, thereby establishing a positive feedback loop to regulate mRNA splicing of its own locus. A small 17-kDa isoform of Periaxin similarly traffics between the cytoplasm and the nucleus to regulate mRNA splicing. Thus, AHNAK constitutes a novel mechanism in post-transcriptional control of gene expression.

Biological activities and molecular interactions of the C-terminal residue of thrombospondin-4, an epitome of acidic amphipathic peptides

C21, the C-terminal residue of thrombospondin-4 (TSP-4), was identified as a peptide growth factor during an investigation concerning erythropoietin-dependent, erythroid stimulating factors of endothelial origin. It is active in cultures of several human hematopoietic stem cells, skin fibroblasts and kidney epithelial cells and stimulates red cell formation in anemic mice. A method of affinity chromatography in the presence of high concentrations of Triton X-100, previously developed for identifying proteins associated with the TSP-1 receptor CD47, was utilized for the detection of C21 binding molecules and their detergent-resistant, associated partners. These experiments helped to delineate two different mechanisms of C21 action, which are compatible with its cell proliferating activity. As a cell matrix peptide, C21 binds to the osteopontin receptor CD44 and could act as an osteopontin antagonist, preventing the inhibition of primitive hematopoietic stem cell proliferation. TSP-1, another matrix protein, binds to C21 and could indirectly act as an antagonist, by shunting C21-CD44 interactions. The second mechanism is a direct effect of C21 on cell proliferation. The extremely rapid internalization and nuclear localization of the peptide could be explained by CD44-mediated internalization, followed by a microtubule-mediated transport towards the nucleus, or, eventually, direct membrane insertion. These alternative hypotheses are supported by previously observed membrane insertion of similar synthetic and viral acidic amphipathic peptides, the presence of microtubule-associated protein 1B (MAP1B) and dynactin in the triton-soluble complexes associated with C21 and the presence in such complexes of dual compartment proteins for nuclei and plasma membranes, such as MAP1B, AHNAK and CD44.

Roles of Ca(v) channels and AHNAK1 in T cells: the beauty and the beast

T lymphocytes require Ca2+ entry though the plasma membrane for their activation and function. Recently, several routes for Ca2+ entry through the T-cell plasma membrane after activation have been described. These include calcium release-activated channels (CRAC), transient receptor potential (TRP) channels, and inositol-1,4,5-trisphosphate receptors (IP3Rs). Herein we review the emergence of a fourth new route for Ca2+ entry, composed of Ca(v) channels (also known as L-type voltage-gated calcium channels) and the scaffold protein AHNAK1 (AHNAK/desmoyokin). Both helper (CD4+) and killer (CD8+) T cells express high levels of Ca(v)1 alpha1 subunits (alpha1S, alpha1C, alpha1D, and alpha1F) and AHNAK1 after their differentiation and require these molecules for Ca2+ entry during an immune response. In this article, we describe the observations and open questions that ultimately suggest the involvement of multiple consecutive routes for Ca2+ entry into lymphocytes, one of which may be mediated by Ca(v) channels and AHNAK1.

Mechanisms of muscle degeneration, regeneration, and repair in the muscular dystrophies

To withstand the rigors of contraction, muscle fibers have specialized protein complexes that buffer against mechanical stress and a multifaceted repair system that is rapidly activated after injury. Genetic studies first identified the mechanosensory signaling network that connects the structural elements of muscle and, more recently, have identified repair elements of muscle. Defects in the genes encoding the components of these systems lead to muscular dystrophy, a family of genetic disorders characterized by progressive muscle wasting. Although the age of onset, affected muscles, and severity vary considerably, all muscular dystrophies are characterized by muscle necrosis that overtakes the regenerative capacity of muscle. The resulting replacement of muscle by fatty and fibrous tissue leaves muscle increasingly weak and nonfunctional. This review discusses the cellular mechanisms that are primarily and secondarily disrupted in muscular dystrophy, focusing on membrane degeneration, muscle regeneration, and the repair of muscle.

The C type natriuretic peptide receptor tethers AHNAK1 at the plasma membrane to potentiate arachidonic acid-induced calcium mobilization

Arachidonic acid (AA) liberated from membrane phospholipids is known to activate phospholipase C gamma1 (PLCgamma1) concurrently with AHNAK in nonneuronal cells. The recruitment of AHNAK from the nucleus is required for it to activate PLCgamma1 at the plasma membrane. Here, we identify the C-type natriuretic peptide receptor (NPR-C), an atypical G protein-coupled receptor, as a protein binding partner for AHNAK1 in various cell types. Mass spectrometry and MASCOT analysis of excised bands from NPR-C immunoprecipitation studies revealed multiple signature peptides corresponding to AHNAK1. Glutathione S-transferase (GST) pulldown assays using GST- AHNAK1 fusion proteins corresponding to each of the distinct domains of AHNAK1 showed the C1 domain of AHNAK1 associates with NPR-C. The role of NPR-C in mediating AA-dependent AHNAK1 calcium signaling was explored in various cell types, including 3T3-L1 preadipocytes during the early stages of differentiation. Sucrose density gradient centrifugation studies showed AHNAK1 resides in the nucleus, cytoplasm, and at the plasma membrane, but small interfering RNA (siRNA)-mediated knockdown of NPR-C resulted in AHNAK1 accumulation in the nucleus. Overexpression of a portion of AHNAK1 resulted in augmentation of intracellular calcium mobilization, whereas siRNA-mediated knockdown of NPR-C or AHNAK1 protein resulted in attenuation of intracellular calcium mobilization in response to phorbol 12-myristate 13-acetate. We characterize the novel association between AHNAK1 and NPR-C and provide evidence that this association potentiates the AA-induced mobilization of intracellular calcium. We address the role of intracellular calcium in the various cell types that AHNAK1 and NPR-C were found to associate.

Evidence that CT694 is a novel Chlamydia trachomatis T3S substrate capable of functioning during invasion or early cycle development

Chlamydia trachomatis is an obligate intracellular parasite, occupies a membrane-bound vacuole throughout development and is capable of manipulating the eukaryotic host by translocating effector molecules via a type III secretion system (T3SS). The infectious chlamydial elementary body (EB) is metabolically inactive yet possesses a functional T3S apparatus capable of translocating effector proteins into the host cell to facilitate invasion and other early cycle events. We present evidence here that the C. trachomatis protein CT694 represents an early cycle-associated effector protein. CT694 is secreted by the Yersinia T3SS and immunodetection studies of infected HeLa cultures indicate that CT694-specific signal accumulates directly adjacent to, but not completely overlapping with EBs during invasion. Yeast two-hybrid analyses revealed an interaction of CT694 with the repeat region and C-terminus of human AHNAK. Immunolocalization studies of CT694 ectopically expressed in HeLa cells were consistent with an interaction with endogenous AHNAK. Additionally, expression of CT694 in HeLa cells resulted in alterations in the detection of stress fibres that correlated with the ability of CT694 to interact with AHNAK. These data indicate that CT694 is a novel T3S-dependent substrate unique to C. trachomatis, and that its interaction with host proteins such as AHNAK may be important for aspects of invasion or development particular to this species.

The giant protein AHNAK involved in morphogenesis and laminin substrate adhesion of myelinating Schwann cells

Within the nervous system, expression of the intriguing giant protein AHNAK had been reported so far only for blood-brain barrier forming vascular endothelium. In a screen for genes upregulated after spinal cord injury, we recently identified ahnak as being highly expressed by non-neuronal cells invading the lesion, delimiting the interior surface of cystic cavities in front of barrier-forming astrocytes. Here, we show for the first time that AHNAK is constitutively expressed in peripheral nervous system, notably by myelinating Schwann cells (SCs), in which we investigated its function. During sciatic nerve development, AHNAK is redistributed from adaxonal toward abaxonal SC compartments in contact with basement membrane. AHNAK labeling on myelinated fibers from adult nerve delineates the so-called "Cajal bands," constituting the residual peripheral SC cytoplasm. Its distribution pattern is complementary to that of periaxin, known to be involved in the myelination process. In vitro, nonconfluent cultured primary SCs seeded on laminin express high levels of AHNAK concentrated in their processes, whereas at confluence, AHNAK is downregulated together with laminin receptor dystroglycan. AHNAK silencing by siRNA interference affects SC morphology and laminin-substrate attachment, as well as expression and distribution of dystroglycan. Thus, our results clearly show the implication of AHNAK in SC adhesion to laminin, probably via targeting of the dystroglycan-associated receptor complex. These findings are of high interest regarding the importance of SC-basal lamina interactions for myelination and myelin maintenance, and open up new perspectives for investigations of the molecular mechanisms underlying demyelinating neuropathies.

Dynamic interactions between L-type voltage-sensitive calcium channel Cav1.2 subunits and ahnak in osteoblastic cells

Voltage-sensitive Ca(2+) channels (VSCCs) mediate Ca(2+) permeability in osteoblasts. Association between VSCC alpha(1)- and beta-subunits targets channel complexes to the plasma membrane and modulates function. In mechanosensitive tissues, a 700-kDa ahnak protein anchors VSCCs to the actin cytoskeleton via the beta(2)-subunit of the L-type Ca(v)1.2 (alpha(1C)) VSCC complex. Ca(v)1.2 is the major alpha(1)-subunit in osteoblasts, but the cytoskeletal complex and subunit composition are unknown. Among the four beta-subtypes, the beta(2)-subunit and, to a lesser extent, the beta(3)-subunit coimmunoprecipitated with the Ca(v)1.2 subunit in MC3T3-E1 preosteoblasts. Fluorescence resonance energy transfer revealed a complex between Ca(v)1.2 and beta(2)-subunits and demonstrated their association in the plasma membrane and secretory pathway. Western blot and immunohistochemistry showed ahnak association with the channel complex in the plasma membrane via the beta(2)-subunit. Cytochalasin D exposure disrupted the actin cytoskeleton but did not disassemble or disrupt the function of the complex of L-type VSCC Ca(v)1.2 and beta(2)-subunits and ahnak. Similarly, small interfering RNA knockdown of ahnak did not disrupt the actin cytoskeleton but significantly impaired Ca(2+) influx. Collectively, we showed that Ca(v)1.2 and beta(2)-subunits and ahnak form a stable complex in osteoblastic cells that permits Ca(2+) signaling independently of association with the actin cytoskeleton.

Cardiac regulation by phosphoinositide 3-kinases and PTEN

The diverse effects mediated by PI3K/PTEN (phosphoinositide 3-kinase/phosphatase and tensin homologue deleted on chromosome 10) signalling in the heart clearly support an important biological and pathophysiological role for this signalling cascade. PI3Ks are a family of evolutionarily conserved lipid kinases that mediate many cellular responses to physiological and pathophysiological stimuli. Class I PI3K can be activated by either receptor tyrosine kinase/cytokine receptor activation (class IA) or G-protein-coupled receptors (class IB), leading to the generation of phosphatidyl inositol (3,4,5)P3 and recruitment and activation of Akt/protein kinase B, 3'-phosphoinositide-dependent kinase-1 (PDK1), or monomeric G-proteins, and phosphorylation of a wide range of downstream targets including glycogen synthase kinase 3beta (GSK3beta), mTOR (mammalian target of rapamycin), p70S6 kinase, endothelial nitric oxide synthase, and several anti-apoptotic effectors. Class IA (PI3Kalpha, beta, and delta) and class IB (PI3Kgamma) PI3Ks mediate distinct phenotypes in the heart under negative control by the 3'-lipid phosphatase PTEN, which dephosphorylates PtdIns(3,4,5)P3 to generate PtdIns(4,5)P2. PI3Kalpha, PI3Kgamma, and PTEN are expressed in cardiomyocytes, fibroblasts, endothelial cells, and vascular smooth muscle cells, where they modulate cell survival, hypertrophy, contractility, metabolism, and mechanotransduction. The PI3K/PTEN signalling pathways are involved in a wide variety of diseases including myocardial hypertrophy and contractility, heart failure, and preconditioning. In this review, we discuss the signalling pathways mediated by PI3K class I isoforms and PTEN and their roles in cardiac structure and function.

AKT pathway in neuroblastoma and its therapeutic implication

Neuroblastoma is a frequent pediatric tumor with a poor outcome in spite of aggressive treatment, even with autologous hematopoietic stem cell transplantation. The overall cure rate of 40% is unsatisfactory and new therapeutic strategies are urgently needed. AKT is a major mediator of survival signals that protect cells from apoptosis and regulate cell proliferation. The AKT signaling network is considered a key determinant of the biological aggressiveness of these tumors. In this article, the authors discuss the relation between activators of AKT in neuroblastoma, in particular, growth factors such as IGF-1, TRK, GDNF, VEGF and EGF, and their effects on tumoral proliferation, differentiation and apoptosis. Numerous other proteins interact with AKT in neuroblastoma. Several are relatively well characterized, such as PTEN and retinoic acid; others are new and potentially interesting, such as PKC and anaplastic lymphoma kinase. Specific inhibition of AKT has been studied, such as with LY249002, with significant effects on cell progression and apoptosis in tumoral cells. Moreover, a series of new drugs, such as geldanamycin and rapamycin, directly modify the expression of AKT in tumoral cells. Few specific inhibitors of AKT are available; less specific inhibitors are probably unsuitable therapeutic options in neuroblastoma. Drugs with a direct or indirect inhibitory effect on the AKT pathway, used alone or in combination with other drugs, seem to hold great promise as a new therapeutic modality in neuroblastoma.

A scaffold protein, AHNAK1, is required for calcium signaling during T cell activation

Engagement of the T cell antigen receptor (TCR) during antigen presentation initiates a coordinated action of a large number of signaling proteins and ion channels. AHNAK1 is a scaffold protein, highly expressed by CD4+ T cells, and is a critical component for calcium signaling. We showed that AHNAK1-deficient mice were highly susceptible to Leishmania major infection. AHNAK1-deficient CD4+ T cells responded poorly to TCR stimulation in vitro with low proliferation and low Interleukin-2 production. Furthermore, AHNAK1 deficiency resulted in a reduced calcium influx upon TCR crosslinking and subsequent poor activation of the transcription factor NFAT. AHNAK1 was required for plasma membrane expression of L-type calcium channels alpha 1S (Cav1.1), probably through its interaction with the beta regulatory subunit. Thus, AHNAK1 plays an essential role in T cell Ca2+ signaling through Cav1 channels, triggered via TCR activation; therefore, AHNAK1 is a potential target for therapeutic intervention.

Identification and validation of eukaryotic aspartate and glutamate methylation in proteins

Methylation of lysine and arginine is known to be critical in cellular processes. However, methylation of other amino acidic residues has been largely overlooked. Here, we report a systematic screening for methylation of side chains of aspartate and glutamate (D/E-methylation), involving exhaustive nano-HPLC/MS/MS, a protein sequence database search, and manual verification. The putative D/E-methylated peptides were confirmed by MS/MS of synthetic peptides. Our analysis identified several D/E-methylation substrate proteins and their modification sites in human and yeast cells. To our knowledge, this is the first report conclusively identifying in vivo D/E-methylation substrates and their modification sites in eukaryotic cells, demonstrating that D/E-methylations are abundant protein modifications. The substrate proteins identified here provide a stepping stone for future biochemical characterization of protein methylation pathways.

Ahnak protein activates protein kinase C (PKC) through dissociation of the PKC-protein phosphatase 2A complex

We have previously reported that central repeated units (CRUs) of Ahnak act as a scaffolding protein networking phospholipase Cgamma and protein kinase C (PKC). Here, we demonstrate that an Ahnak derivative consisting of four central repeated units binds and activates PKC-alpha in a phosphatidylserine/1,2-dioleoyl-sn-glycerol-independent manner. Moreover, NIH3T3 cells expressing the 4 CRUs of Ahnak showed enhanced c-Raf, MEK, and Erk phosphorylation in response to phorbol 12-myristate 13-acetate (PMA) compared with parental cells. To evaluate the effect of loss-of-function of Ahnak in cell signaling, we investigated PKC activation and Raf phosphorylation in embryonic fibroblast cells (MEFs) of the Ahnak knock-out (Ahnak(-/-)) mouse. Membrane translocation of PKC-alpha and phosphorylation of Raf in response to PMA or platelet-derived growth factor were decreased in Ahnak null MEF cells compared with wild type MEFs. Several lines of evidence suggest that PKC-alpha activity is regulated through association with protein phosphatase 2A (PP2A). A co-immunoprecipitation assay indicated that the association of PKC-alpha with PP2A was disrupted in NIH3T3 cells expressing 4 CRUs of Ahnak in response to PMA. Consistently, Ahnak null MEF cells stimulated by PMA showed enhanced PKC-PP2A complex formation, and add-back expression of Ahnak into Ahnak null MEF cells abolished the PKC-PP2A complex formation in response to PMA. These data indicate that Ahnak potentiates PKC activation through inhibiting the interaction of PKC with PP2A.

An emerin "proteome": purification of distinct emerin-containing complexes from HeLa cells suggests molecular basis for diverse roles including gene regulation, mRNA splicing, signaling, mechanosensing, and nuclear architecture

Using recombinant bead-conjugated emerin, we affinity-purified seven proteins from HeLa cell nuclear lysates that bind emerin either directly or indirectly. These proteins were identified by mass spectrometry as nuclear alphaII-spectrin, nonmuscle myosin heavy chain alpha, Lmo7 (a predicted transcription regulator; reported separately), nuclear myosin I, beta-actin (reported separately), calponin 3, and SIKE. We now report that emerin binds nuclear myosin I (NMI, a molecular motor) directly in vitro. Furthermore, bead-conjugated emerin bound nuclear alphaII-spectrin and NMI equally well with or without ATP (which stimulates motor activity), whereas ATP decreased actin binding by 65%. Thus alphaII-spectrin and NMI interact stably with emerin. To investigate the physiological relevance of these interactions, we used antibodies against emerin to affinity-purify emerin-associated protein complexes from HeLa cells and then further purified by ion-exchange chromatography to resolve by net charge and by size exclusion chromatography yielding six distinct emerin-containing fractions (0.5-1.6 MDa). Western blotting suggested that each complex had distinct components involved in nuclear architecture (e.g., NMI, alphaII-spectrin, lamins) or gene or chromatin regulation (BAF, transcription regulators, HDACs). Additional constituents were identified by mass spectrometry. One putative gene-regulatory complex (complex 32) included core components of the nuclear corepressor (NCoR) complex, which mediates gene regulation by thyroid hormone and other nuclear receptors. When expressed in HeLa cells, FLAG-tagged NCoR subunits Gps2, HDAC3, TBLR1, and NCoR each co-immunoprecipitated emerin, validating one putative complex. These findings support the hypothesis that emerin scaffolds a variety of functionally distinct multiprotein complexes at the nuclear envelope in vivo. Notably included are nuclear myosin I-containing complexes that might sense and regulate mechanical tension at the nuclear envelope.

Annexin2 coating the surface of enlargeosomes is needed for their regulated exocytosis

Enlargeosomes are small cytoplasmic vesicles that undergo rapid, Ca2+-dependent exo/endocytosis. The role of the cytoskeleton in these processes was unknown. In PC12-27 cells, microtubule disassembly had little effect on enlargeosomes, whereas microfilament disassembly increased markedly both their resting and stimulated exocytosis, and inhibited their endocytosis. Even at rest enlargeosomes are coated at their cytosolic surface by an actin-associated protein, annexin2, bound by a dual, Ca2+-dependent and Ca2+-independent mechanism. In contrast, the other enlargeosome marker, desmoyokin/Ahnak, is transported across the organelle membrane, apparently by an ABC transporter, and binds to its lumenal face. Annexin2-GFP expression revealed that, upon stimulation, the slow and random enlargeosome movement increases markedly and becomes oriented toward the plasma membrane. After annexin2 downregulation enlargeosome exocytosis induced by both [Ca2+]i rise and cytoskeleton disruption is inhibited, and the NGF-induced differentiation is blocked. Binding of annexin2 to the enlargeosome membrane, the most extensive ever reported (>50% annexin2 bound to approximately 3% of total membrane area), seems therefore to participate in the regulation of their exocytosis.

A retrovirus-based protein complementation assay screen reveals functional AKT1-binding partners

We developed a retrovirus-based protein-fragment complementation assay (RePCA) screen to identify protein-protein interactions in mammalian cells. In RePCA, bait protein is fused to one fragment of a rationally dissected fluorescent protein, such as GFP, intensely fluorescent protein, or red fluorescent protein. The second, complementary fragment of the fluorescent protein is fused to an endogenous protein by in-frame exon traps in the enhanced retroviral mutagen vector. An interaction between bait and host protein (prey) places the two parts of the fluorescent molecule in proximity, resulting in reconstitution of fluorescence. By using RePCA, we identified a series of 24 potential interaction partners or substrates of the serine/threonine protein kinase AKT1. We confirm that alpha-actinin 4 (ACTN4) interacts physically and functionally with AKT1. siRNA-mediated ACTN4 silencing down-regulates AKT phosphorylation, blocks AKT translocation to the membrane, increases p27(Kip1) levels, and inhibits cell proliferation. Thus, ACTN4 is a critical regulator of AKT1 localization and function.

Spinal cord injury-induced up-regulation of AHNAK, expressed in cells delineating cystic cavities, and associated with neoangiogenesis

To investigate the molecular basis for the poor regenerative capacity of the mammalian central nervous system (CNS) after injury, we searched for genes whose expression was affected by an adult rat spinal cord hemi-section. Differential screening of a rat spinal cord expression library was performed using polyclonal antibodies raised against lesioned spinal cord tissue. A striking overexpression was found for ahnak, encoding a 700-kDa protein, in normal CNS present only in the blood-brain barrier (BBB) forming vascular endothelial cells. Indeed, very early after spinal cord injury (SCI), high levels of membrane-associated AHNAK are observed on non-neuronal cells invading the lesion site. With time, AHNAK distribution spreads rostrally and caudally concomitant with the process of tissue inflammation and axon degeneration, delineating the interior surface of cystic cavities, mainly in front of barrier-forming astrocytes. Strong overexpression is also observed on vascular endothelial cells reacting to BBB breakdown. Based on our detailed analysis of its spatiotemporal and cellular expression, and its previously described function in BBB, we suggest that AHNAK expression is associated with cell types displaying tissue-protective barrier properties. Our study may thus contribute to the elucidation of the precise molecular and cellular events that eventually render traumatic spinal cord tissue non-permissive for regeneration.

Cell growth-dependent subcellular localization of p8

p8 is a stress-induced protein, biochemically related to the architectural factor HMG-I/Y, overexpressed in many cancers and required for tumor expansion. The molecular mechanisms by which p8 may exert its effect in aspects of growth is unknown. Using immunocytochemistry, we found that p8 presents nuclear localization in sub-confluent cells, but it localizes throughout the whole cell in high density grown cells. Cells arrested in Go/G1, either by serum deprivation or by hydroxyurea treatment, show a nucleo-cytoplasmic localization of p8, whether in the rest of the cell cycle stages of actively dividing cells the localization is nuclear. A comparison of p8 sequences from human to fly predicts a conserved bipartite nuclear localization sequence (NLS). The putative NLS has been demonstrated to be functional, since nuclear import is energy dependent (inhibited by sodium azide plus 2-deoxyglucose), and fusion proteins GFP-p8 and GFP-NLSp8 localize to the nucleus, whereas GFP-p8NLSmut in which with Lys 65, 69, 76, and 77 mutated to Ala localized to the whole cell. p8 localization does not involve the CRM1 transporter, since it is insensitive to leptomycin B. Inhibitors of MAPK pathways did not affect p8 subcellular localization. The inhibition of deacetylation with Trichostatin A promotes cytoplasmic accumulation of p8. The results suggest that p8 growth stage-dependent localization is regulated by acetylation, that p8 is not free within the cell but forming part of a complex and that it may exert a role in both subcellular localizations.

Highly purified lipoteichoic acid from gram-positive bacteria induces in vitro blood–brain barrier disruption through glia activation: Role of pro-inflammatory cytokines and nitric oxide

The co-culture of bovine brain capillary endothelial cells and rat primary glial cells was established as an in vitro blood-brain barrier model to investigate the mechanisms by which the Gram-positive bacterial cell wall components lipoteichoic acid and muramyl dipeptide induced injury of blood-brain barrier structure and function. We found that highly purified lipoteichoic acid disrupted blood-brain barrier integrity in a concentration- and time-dependent manner indirectly, through glia activation. Low trans-endothelial electrical resistance and high permeability to fluorescein isothiocyanate-inulin observed in the presence of lipoteichoic acid-activated glial cells were potentiated by muramyl dipeptide and could be reversed only when glial cells were activated by lipoteichoic acid at 10 microg/ml but not with a higher lipoteichoic acid concentration (30 microg/ml). Immunocytochemistry analysis revealed no evident changes in the distribution of the cytoskeleton protein F-actin and tight junction proteins occludin and claudin after lipoteichoic acid treatment. However, the tight junction associated protein AHNAK clearly revealed the morphological alteration of the endothelial cells induced by lipoteichoic acid. Lipoteichoic acid-activated glial cells produced nitric oxide and pro-inflammatory cytokines (tumor necrosis factor-alpha and interleukin-1beta) that contributed to lipoteichoic acid-induced blood-brain barrier disruption, since the direct treatment of the endothelial monolayer with tumor necrosis factor-alpha or interleukin-1beta increased blood-brain barrier permeability, whereas the pre-treatment of lipoteichoic acid-activated glial cells with antibodies against these two cytokines blocked lipoteichoic acid effects. Additionally, nitric oxide was also involved in blood-brain barrier damage, since the nitric oxide donor itself (diethylenetriamine-nitric oxide adduct) increased blood-brain barrier permeability and inducible nitric oxide synthase inhibitor (1400W) partially reversed lipoteichoic acid-induced trans-endothelial electrical resistance decrease.

Ahnak is critical for cardiac Ca(v)1.2 calcium channel function and its β‐adrenergic regulation

Defective L-type Ca2+ channel (I(CaL)) regulation is one major cause for contractile dysfunction in the heart. The I(CaL) is enhanced by sympathetic nervous stimulation: via the activation of beta-adrenergic receptors, PKA phosphorylates the alpha1C(Ca(V)1.2)- and beta2-channel subunits and ahnak, an associated 5643-amino acid (aa) protein. In this study, we examined the role of a naturally occurring, genetic variant Ile5236Thr-ahnak on I(CaL). Binding experiments with ahnak fragments (wild-type, Ile5236Thr mutated) and patch clamp recordings revealed that Ile5236Thr-ahnak critically affected both beta2 subunit interaction and I(CaL) regulation. Binding affinity between ahnak-C1 (aa 4646-5288) and beta2 subunit decreased by approximately 50% after PKA phosphorylation or in the presence of Ile5236Thr-ahnak peptide. On native cardiomyocytes, intracellular application of this mutated ahnak peptide mimicked the PKA-effects on I(CaL) increasing the amplitude by approximately 60% and slowing its inactivation together with a leftward shift of its voltage dependency. Both mutated Ile5236Thr-peptide and Ile5236Thr-fragment (aa 5215-5288) prevented specifically the further up-regulation of I(CaL) by isoprenaline. Hence, we suggest the ahnak-C1 domain serves as physiological brake on I(CaL). Relief from this inhibition is proposed as common pathway used by sympathetic signaling and Ile5236Thr-ahnak fragments to increase I(CaL). This genetic ahnak variant might cause individual differences in I(CaL) regulation upon physiological challenges or therapeutic interventions.

Identification of a nuclear export signal in the KSHV latent protein LANA2 mediating its export from the nucleus

LANA2 is a latent protein detected in Kaposi's sarcoma-associated herpesvirus (KSHV)-infected B cells that inhibits p53-dependent transcriptional transactivation and apoptosis and PKR-dependent apoptosis, suggesting an important role in the transforming activity of the virus. It has been reported that LANA2 localizes into the nucleus of both KSHV-infected B cells and transiently transfected HeLa cells. In this study, we show that LANA2 is a nucleocytoplasmic shuttling protein that requires a Rev-type nuclear export signal located in the C-terminus to direct the protein to the cytoplasm, through an association with the export receptor CRM1. In addition, a functional protein kinase B (PKB)/Akt phosphorylation motif partially overlapping with the nuclear export signal was identified. Nuclear exclusion of LANA2 was negatively regulated by the phosphorylation of threonine 564 by Akt. The ability of LANA2 to shuttle between nucleus and cytoplasm has implications for the function of this viral protein.

Induction of blood-brain barrier properties in cultured brain capillary endothelial cells: comparison between primary glial cells and C6 cell line

The communication between glial cells and brain capillary endothelial cells is crucial for a well-differentiated blood-brain barrier (BBB). It has been suggested that in vitro primary glial cells (GCs) be replaced by the glial C6 cell line to standardise the model further. This study compares directly the structural and functional differentiation of bovine brain capillary endothelial cells (BBCECs) induced by co-culture with rat primary GCs or C6 cells, for the first time. Trans-endothelial electrical resistance (TEER) measurements showed that under no condition were C6 cells able to reproduce TEER values as high as in the presence of GCs. At the same time, permeability of the BBCECs to both radioactive sucrose and FITC-inulin was 2.5-fold higher when cells were co-cultured with C6 than with GCs. Furthermore, immunocytochemistry studies showed different cell morphology and less developed tight junction pattern of BBCECs co-cultured with C6 toward GCs. Additionally, studies on P-glycoprotein (P-gp) showed much lower P-gp presence and activity in BBCECs co-cultured with C6 than GCs. Both VEGF mRNA expression and protein content were dramatically increased when compared with GCs, suggesting that VEGF could be one of the factors responsible for higher permeability of BBB. Our results clearly indicate that, in the presence of the glial C6 cell line, BBCECs did not differentiate as well as in the co-culture with primary GCs at both structural and functional levels.

Specific AHNAK expression in brain endothelial cells with barrier properties

The blood-brain barrier (BBB) is essential for maintaining brain homeostasis and low permeability. Because disruption of the BBB may contribute to many brain disorders, they are of considerable interests in the identification of the molecular mechanisms of BBB development and integrity. We here report that the giant protein AHNAK is expressed at the plasma membrane of endothelial cells (ECs) forming specific blood-tissue barriers, but is absent from the endothelium of capillaries characterized by extensive molecular exchanges between blood and extracellular fluid. In the brain, AHNAK is widely distributed in ECs with BBB properties, where it co-localizes with the tight junction protein ZO-1. AHNAK is absent from the permeable brain ECs of the choroid plexus and is down-regulated in permeable angiogenic ECs of brain tumors. In the choroid plexus, AHNAK accumulates at the tight junctions of the choroid epithelial cells that form the blood-cerebrospinal fluid (CSF) barrier. In EC cultures, the regulation of AHNAK expression and its localization corresponds to general criteria of a protein involved in barrier organization. AHNAK is up-regulated by angiopoietin-1 (Ang-1), a morphogenic factor that regulates brain EC permeability. In bovine cerebral ECs co-cultured with glial cells, AHNAK relocates from the cytosol to the plasma membrane when endothelial cells acquire BBB properties. Our results identify AHNAK as a protein marker of endothelial cells with barrier properties.

Ahnak/Desmoyokin is dispensable for proliferation, differentiation, and maintenance of integrity in mouse epidermis

Desmoyokin was first isolated from bovine muzzle epidermis and thought to be an epidermal desmosome-related protein. We previously demonstrated that the Desmoyokin gene is identical to the Ahnak gene, which is expressed ubiquitously and downregulated in neuroblastomas. It was assumed Ahnak/Desmoyokin was associated with epidermal cell adhesion, tumorigenesis, cell proliferation and differentiation, and embryonic development. To determine the precise biological function of Ahnak/Desmoyokin, we generated a null mutation in ES cells and mice. The resultant Ahnak/Desmoyokin-deficient ES cells normally differentiated into embryoid bodies and neural cells. The mutant mice were viable and fertile and showed no gross developmental defects. Electron microscopic examination of skin sections demonstrated that the ultrastructure of epidermal intercellular junctions, including desmosomes, of the mutant mice was indistinguishable from that of wild-type mice. Two-stage chemical skin carcinogenesis experiments showed no difference in frequency or onset of cutaneous tumor formation between wild-type and mutant mice. Moreover, no tumorigenesis was observed in other tissues and organs of mutant mice up to 2 y of age. These results lead us to conclude that Ahnak/Desmoyokin deficiency has only a minimal effect on epidermal cell adhesion, tumorigenesis, cell proliferation and differentiation, and overall mouse development.