Revealing the spatiotemporal brain dynamics of covert speech compared with overt speech: A simultaneous EEG-fMRI study.

Covert speech (CS) refers to speaking internally to oneself without producing any sound or movement. CS is involved in multiple cognitive functions and disorders. Reconstructing CS content by brain-computer interface (BCI) is also an emerging technique. However, it is still controversial whether CS is a truncated neural process of overt speech (OS) or involves independent patterns. Here, we performed a word-speaking experiment with simultaneous EEG-fMRI. It involved 32 participants, who generated words both overtly and covertly. By integrating spatial constraints from fMRI into EEG source localization, we precisely estimated the spatiotemporal dynamics of neural activity. During CS, EEG source activity was localized in three regions: the left precentral gyrus, the left supplementary motor area, and the left putamen. Although OS involved more brain regions with stronger activations, CS was characterized by an earlier event-locked activation in the left putamen (peak at 262 ms versus 1170 ms). The left putamen was also identified as the only hub node within the functional connectivity (FC) networks of both OS and CS, while showing weaker FC strength towards speech-related regions in the dominant hemisphere during CS. Path analysis revealed significant multivariate associations, indicating an indirect association between the earlier activation in the left putamen and CS, which was mediated by reduced FC towards speech-related regions. These findings revealed the specific spatiotemporal dynamics of CS, offering insights into CS mechanisms that are potentially relevant for future treatment of self-regulation deficits, speech disorders, and development of BCI speech applications.

West Nile virus and dengue virus capsid protein negates the antiviral activity of human Sec3 protein through the proteasome pathway.

Flavivirus capsid (C) protein is a key structural component of virus particles. The non-structural role of C protein in the pathogenesis of arthropod-borne flaviviruses is not clearly deciphered. This study showed that West Nile virus (WNV) and dengue virus (DENV) utilized C protein to reduce human Sec3p (hSec3p) levels at post-transcriptional level through activation of chymotrypsin-like proteolytic function of 20S proteasome. Mutagenesis studies confirmed amino acids 14, 109-114 of WNV C protein and 13, 102-107 of DENV C protein played an important role in activating the proteolytic function of 20S proteasome. Amino acid residues at 14 (WNV) and 13 (DENV) of C protein were important for C protein-hSec3p binding and physical interaction between C protein and hSec3p was essential to execute hSec3p degradation. Degradation motif required to degrade hSec3p resided between amino acid residues 109-114 of WNV C protein and 102-107 of DENV C protein. Proteasomes, hSec3p binding motif and degradation motif on C protein must be intact for efficient flavivirus production. Clinical isolates of DENV showed more pronounced effect in manipulating the proteasomes and reducing hSec3p levels. This study portrayed the non-structural function of C protein that helped the flavivirus to nullify the antiviral activity of hSec3p by accelerating its degradation and facilitating efficient binding of elongation factor 1α with flaviviral RNA genome.

West Nile virus capsid protein interaction with importin and HDM2 protein is regulated by protein kinase C-mediated phosphorylation.

West Nile virus (WNV) capsid (C) protein was shown to enter the nucleus via importin-mediated pathway and induce apoptosis although the precise regulatory mechanisms for such events have remained elusive. In this study, it was shown that WNV C protein was phosphorylated by protein kinase C (PKC). PKC-mediated phosphorylation influenced nuclear trafficking of C protein by modulating the efficiency of C protein-importin-alpha binding. Combination of bio-informatics, site-directed mutagenesis, co-immunoprecipitation, immuno-fluorescence and mammalian two-hybrid analyses showed that phosphorylation at amino acid residues residing near (Ser83) or within (Ser99 and Thr100) the bipartite nuclear localization motif of WNV C protein was essential for efficient interaction between C protein and importin-alpha. In addition, phosphorylation of WNV C protein by PKC was shown to enhance its binding to HDM2 and could subsequently induce p53-dependent apoptosis. Collectively, this study highlighted that phosphorylation is an important post-translational modification required to execute the functions of C protein.

Human Sec3 protein is a novel transcriptional and translational repressor of flavivirus.

The Flaviviridae family consists of several medically important pathogens such as West Nile virus (WNV) and Dengue virus (DENV). Flavivirus capsid (C) protein is a key structural component of virus particles. However, the role of C protein in the pathogenesis of arthropod-borne flaviviruses is poorly understood. To examine whether flavivirus C protein can associate with cellular proteins, and contribute to viral pathogenesis, WNV/DENV C protein was screened against a human brain/liver cDNA yeast two-hybrid library. This study identified human Sec3 exocyst protein (hSec3p) as a novel interacting partner of WNV and DENV C protein. Mutagenesis studies showed that the SH2 domain-binding motif of hSec3p binds to the first 15 amino acids of C protein. We report for the first time that hSec3p can modulate virus production by affecting viral RNA transcription and translation through the sequestration of elongation factor 1alpha (EF1alpha). This molecular discovery shed light on the protective role of hSec3p during flavivirus infection. This study also highlighted the antagonistic mechanism adopted by flavivirus C protein that can negatively regulate the formation of hSec3p-EF1alpha complex by sequestering hSec3p to establish successful infection.

Specific interaction of capsid protein and importin-alpha/beta influences West Nile virus production.

West Nile virus (WNV) capsid (C) protein has been shown to enter the nucleus of infected cells. However, the mechanism by which C protein enters the nucleus is unknown. In this study, we have unveiled for the first time that nuclear transport of WNV and Dengue virus C protein is mediated by their direct association with importin-alpha. This interplay is mediated by the consensus sequences of bipartite nuclear localization signal located between amino acid residues 85-101 together with amino acid residues 42 and 43 of C protein. Elucidation of biological significance of importin-alpha/C protein interaction demonstrated that the binding efficiency of this association influenced the nuclear entry of C protein and virus production. Collectively, this study illustrated the molecular mechanism by which the C protein of arthropod-borne flavivirus enters the nucleus and showed the importance of importin-alpha/C protein interaction in the context of flavivirus life-cycle.

Tyrosine 78 of premembrane protein is essential for assembly of West Nile virus.

Flavivirus premembrane (prM) protein plays an important role in conformational folding of the envelope (E) protein and protects it against premature fusion in acidic vesicles of the Golgi network. Currently, molecular determinants on the prM protein ectodomain which mediate critical steps during the flavivirus assembly process are poorly characterized. In this study, bioinformatics analysis and alanine scanning mutagenesis showed that the amino acid triplet valine 76, tyrosine 78 and glycine 79 is absolutely conserved among flavivirus prM ectodomains. Triple mutations engineered at these residues in prM ectodomain of West Nile virus (WNV) completely abrogated virus infectivity. Site-directed mutagenesis of prM protein revealed that tyrosine 78 of the amino acid triplet was required for virus infectivity and secretion. The mutation did not affect folding, post-translational modifications and trafficking of the prM and E proteins. Ultrastructural studies using transmission electron microscopy confirmed that virus particle formation was blocked by tyrosine 78 mutation. Specificity of assembly defect conferred by tyrosine 78 mutation was demonstrated by positive and negative trans complementation studies. Collectively, these results defined tyrosine 78 as a novel critical determinant present on prM protein ectodomain that is required for flavivirus assembly. Molecular dissection of prM protein function provides the crucial knowledge much needed in the elucidation of flavivirus particle formation.

Recombinant non-structural 1 (NS1) protein of dengue-2 virus interacts with human STAT3beta protein.

A combination of yeast two-hybrid library screening, co-immunoprecipitation and immunofluorescence microscopy demonstrated that dengue-2 virus non-structural 1 (NS1) protein can interact with an N-terminally truncated form of human STAT3beta (DeltaN40-STAT3beta) protein. The NS1 protein interacted with the activated STAT3beta protein in vesicle-like structures in the cell cytoplasm. In addition, transfection of dendritic cells with plasmid expressing NS1 protein also resulted in significant induction of tumor necrosis factor-alpha (TNFalpha) and interleukin-6 (IL-6). Since the STAT3beta protein is an acute-phase response factor, its interaction with NS1 protein may influence the pathological changes observed in dengue fever, dengue hemorrhagic fever and dengue shock syndrome.

Analysis of self-association of West Nile virus capsid protein and the crucial role played by Trp 69 in homodimerization.

The understanding of capsid (C) protein interactions with itself would provide important data on how the core is organized in flaviviruses during assembly. In this study, West Nile (WN) virus C protein was shown to form homodimers using yeast two-hybrid analysis in conjunction with mammalian two-hybrid and in vivo co-immunoprecipitation assays. To delineate the region on the C protein which mediates C-C dimerization, truncation studies were carried out. The results obtained clearly showed that the internal hydrophobic segment flanked by helix I and helix III of WN virus C protein is essential for the self-association of C protein. The crucial role played by Trp 69 in stabilizing the self-association of C protein was also demonstrated by mutating Trp to Gly/Arg/Phe. Substitution of the Trp residue with Gly/Arg abolished the dimerization, whereas substitution with Phe decreased the self-association significantly. The results of this study pinpoint a critical residue in the C protein that potentially plays a role in stabilizing the homotypic interaction.