Expression, phosphorylation, and mRNA-binding of heterogeneous nuclear ribonucleoprotein K in Xenopus oocytes, eggs, and early embryos

Keratin 19 binds and regulates cytoplasmic HNRNPK mRNA targets in triple-negative breast cancer

BACKGROUND

Heterogeneous nuclear ribonucleoprotein K (HNRNPK) regulates pre-mRNA processing and long non-coding RNA localization in the nucleus. It was previously shown that shuttling of HNRNPK to the cytoplasm promotes cell proliferation and cancer metastasis. However, the mechanism of HNRNPK cytoplasmic localization, its cytoplasmic RNA ligands, and impact on post-transcriptional gene regulation remain uncharacterized.

RESULTS

Here we show that the intermediate filament protein Keratin 19 (K19) directly interacts with HNRNPK and sequesters it in the cytoplasm. Correspondingly, in K19 knockout breast cancer cells, HNRNPK does not localize in the cytoplasm, resulting in reduced cell proliferation. We comprehensively mapped HNRNPK binding sites on mRNAs and showed that, in the cytoplasm, K19-mediated HNRNPK-retention increases the abundance of target mRNAs bound to the 3' untranslated region (3'UTR) at the expected cytidine-rich (C-rich) sequence elements. Furthermore, these mRNAs protected by HNRNPK in the cytoplasm are typically involved in cancer progression and include the p53 signaling pathway that is dysregulated upon HNRNPK knockdown (HNRNPK KD) or K19 knockout (KRT19 KO).

CONCLUSIONS

This study identifies how a cytoskeletal protein can directly regulate gene expression by controlling the subcellular localization of RNA-binding proteins to support pathways involved in cancer progression.

The RNA-Binding Protein hnRNP K Mediates the Effect of BDNF on Dendritic mRNA Metabolism and Regulates Synaptic NMDA Receptors in Hippocampal Neurons

Brain-derived neurotrophic factor (BDNF) is an important mediator of long-term synaptic potentiation (LTP) in the hippocampus. The local effects of BDNF depend on the activation of translation activity, which requires the delivery of transcripts to the synapse. In this work, we found that neuronal activity regulates the dendritic localization of the RNA-binding protein heterogeneous nuclear ribonucleoprotein K (hnRNP K) in cultured rat hippocampal neurons by stimulating BDNF-Trk signaling. Microarray experiments identified a large number of transcripts that are coimmunoprecipitated with hnRNP K, and about 60% of these transcripts are dissociated from the protein upon stimulation of rat hippocampal neurons with BDNF. In vivo studies also showed a role for TrkB signaling in the dissociation of transcripts from hnRNP K upon high-frequency stimulation (HFS) of medial perforant path-granule cell synapses of male rat dentate gyrus (DG). Furthermore, treatment of rat hippocampal synaptoneurosomes with BDNF decreased the coimmunoprecipitation of hnRNP K with mRNAs coding for glutamate receptor subunits, Ca²⁺- and calmodulin-dependent protein kinase IIβ (CaMKIIβ) and BDNF. Downregulation of hnRNP K impaired the BDNF-induced enhancement of NMDA receptor (NMDAR)-mediated mEPSC, and similar results were obtained upon inhibition of protein synthesis with cycloheximide. The results demonstrate that BDNF regulates specific populations of hnRNP-associated mRNAs in neuronal dendrites and suggests an important role of hnRNP K in BDNF-dependent forms of synaptic plasticity.

HuR and GRSF1 modulate the nuclear export and mitochondrial localization of the lncRNA RMRP

Noh et al. found two RNA-binding proteins (RBPs)—HuR and GRSF1—that associated with the nuclear DNA-encoded lncRNA RMRP and mobilized it to mitochondria. In cultured human cells, HuR bound RMRP in the nucleus and mediated its CRM1-dependent export to the cytosol. After RMRP was imported into mitochondria, GRSF1 bound RMRP and increased its abundance in the matrix.

Some mitochondrial long noncoding RNAs (lncRNAs) are encoded by nuclear DNA, but the mechanisms that mediate their transport to mitochondria are poorly characterized. Using affinity RNA pull-down followed by mass spectrometry analysis, we found two RNA-binding proteins (RBPs), HuR (human antigen R) and GRSF1 (G-rich RNA sequence-binding factor 1), that associated with the nuclear DNA-encoded lncRNA RMRP and mobilized it to mitochondria. In cultured human cells, HuR bound RMRP in the nucleus and mediated its CRM1 (chromosome region maintenance 1)-dependent export to the cytosol. After RMRP was imported into mitochondria, GRSF1 bound RMRP and increased its abundance in the matrix. Loss of GRSF1 lowered the mitochondrial levels of RMRP, in turn suppressing oxygen consumption rates and modestly reducing mitochondrial DNA replication priming. Our findings indicate that RBPs HuR and GRSF1 govern the cytoplasmic and mitochondrial localization of the lncRNA RMRP, which is encoded by nuclear DNA but has key functions in mitochondria.

Expression and localization of heterogeneous nuclear ribonucleoprotein K in mouse ovaries and preimplantation embryos

Heterogeneous nuclear ribonucleoprotein K (hnRNP K), an evolutionarily conserved protein, is involved in several important cellular processes that are relevant to cell proliferation, differentiation, apoptosis, and cancer development. However, details of hnRNP K expression during mammalian oogenesis and preimplantation embryo development are lacking. The present study investigates the expression and cellular localization of K protein in the mouse ovaries and preimplantation embryos using immunostaining. We demonstrate, for the first time, that hnRNP K is abundantly expressed in the nuclei of mouse oocytes in primordial, primary and secondary follicles. In germ vesicle (GV)-stage oocytes, hnRNP K accumulates in the germinal vesicle in a spot distribution manner. After germinal vesicle breakdown, speckled hnRNP K is diffusely distributed in the cytoplasm. However, after fertilization, the K protein relocates into the female and male pronucleus and persists in the blastomere nuclei. Localization of K protein in the human ovary and ovarian granulosa cell tumor (GCT) was also investigated. Overall, this study provides important morphological evidence to better understand the possible roles of hnRNP K in mammalian oogenesis and early embryo development.

Transmembrane signal transduction in oocyte maturation and fertilization: focusing on Xenopus laevis as a model animal

Fertilization is a cell biological phenomenon of crucial importance for the birth of new life in a variety of multicellular and sexual reproduction species such as algae, animal and plants. Fertilization involves a sequence of events, in which the female gamete "egg" and the male gamete "spermatozoon (sperm)" develop, acquire their functions, meet and fuse with each other, to initiate embryonic and zygotic development. Here, it will be briefly reviewed how oocyte cytoplasmic components are orchestrated to undergo hormone-induced oocyte maturation and sperm-induced activation of development. I then review how sperm-egg membrane interaction/fusion and activation of development in the fertilized egg are accomplished and regulated through egg coat- or egg plasma membrane-associated components, highlighting recent findings and future directions in the studies using Xenopus laevis as a model experimental animal.

The RNA binding protein hnRNP-K mediates post-transcriptional regulation of uncoupling protein-2 by angiopoietin-1

Angiopoietin-1 (Ang1) is a ligand for the receptor tyrosine kinase Tie2 and has key roles in the development of the vascular system and vascular protection. In a screen to define signalling pathways regulated by Ang1 in endothelial cells we found the RNA-binding protein hnRNP-K to be phosphorylated in response to Ang1. The ligand stimulated both tyrosine phosphorylation of hnRNP-K and recruitment of the tyrosine kinase Src to the RNA-binding protein. In endothelial cells hnRNP-K was found bound to mRNA encoding the mitochondrial protein uncoupling protein-2 (UCP2). Ang1 stimulation of cells resulted in the release of UCP2 mRNA from hnRNP-K. Using in vitro assays we confirmed direct binding between hnRNP-K and UCP2 mRNA. Furthermore Src induced phosphorylation of purified hnRNP-K and prevented UCP2 mRNA binding. Tyrosine 458 in the RNA-binding protein was found to be required for suppression of UCP2 mRNA binding by Src phosphorylation. In addition to releasing UCP2 mRNA from hnRNP-K, Ang1 induced an increase in UCP2 protein expression in endothelial cells without affecting total UCP2 mRNA levels. Consistent with the known effects of UCP2 to suppress generation of reactive oxygen species, Ang1 limited ROS production in endothelium stimulated with tumour necrosis factor-α. Taken together these data suggest that UCP2 mRNA is present in endothelial cells bound to hnRNP-K, which holds it in a translationally inactive state, and that Ang1 stimulates Src interaction with hnRNP-K, phosphorylation of the RNA-binding protein, release of these transcripts and upregulation of UCP2 protein expression. This study demonstrates a new mechanism for post-transcriptional regulation of UCP2 by the vascular protective ligand Ang1. The ability to rapidly upregulate UCP2 protein expression may be important in protecting endothelial cells from excessive generation of potentially damaging reactive oxygen species.

Accumulated SET protein up-regulates and interacts with hnRNPK, increasing its binding to nucleic acids, the Bcl-xS repression, and cellular proliferation

SET and hnRNPK are proteins involved in gene expression and regulation of cellular signaling. We previously demonstrated that SET accumulates in head and neck squamous cell carcinoma (HNSCC); hnRNPK is a prognostic marker in cancer. Here, we postulate that SET and hnRNPK proteins interact to promote tumorigenesis. We performed studies in HEK293 and HNSCC (HN6, HN12, and HN13) cell lines with SET/hnRNPK overexpression and knockdown, respectively. We found that SET and/or hnRNPK protein accumulation increased cellular proliferation. SET accumulation up-regulated hnRNPK mRNA and total/phosphorylated protein, promoted hnRNPK nuclear location, and reduced Bcl-x mRNA levels. SET protein directly interacted with hnRNPK, increasing both its binding to nucleic acids and Bcl-xS repression. We propose that hnRNPK should be a new target of SET and that SET-hnRNPK interaction, in turn, has potential implications in cell survival and malignant transformation.

Heterogeneous nuclear ribonucleoprotein k interacts with Abi-1 at postsynaptic sites and modulates dendritic spine morphology

Background

Abelson-interacting protein 1 (Abi-1) plays an important role for dendritic branching and synapse formation in the central nervous system. It is localized at the postsynaptic density (PSD) and rapidly translocates to the nucleus upon synaptic stimulation. At PSDs Abi-1 is in a complex with several other proteins including WASP/WAVE or cortactin thereby regulating the actin cytoskeleton via the Arp 2/3 complex.

Principal Findings

We identified heterogeneous nuclear ribonucleoprotein K (hnRNPK), a 65 kDa ssDNA/RNA-binding-protein that is involved in multiple intracellular signaling cascades, as a binding partner of Abi-1 at postsynaptic sites. The interaction with the Abi-1 SH3 domain is mediated by the hnRNPK-interaction (KI) domain. We further show that during brain development, hnRNPK expression becomes more and more restricted to granule cells of the cerebellum and hippocampal neurons where it localizes in the cell nucleus as well as in the spine/dendritic compartment. The downregulation of hnRNPK in cultured hippocampal neurons by RNAi results in an enlarged dendritic tree and a significant increase in filopodia formation. This is accompanied by a decrease in the number of mature synapses. Both effects therefore mimic the neuronal morphology after downregulation of Abi-1 mRNA in neurons.

Conclusions

Our findings demonstrate a novel interplay between hnRNPK and Abi-1 in the nucleus and at synaptic sites and show obvious similarities regarding both protein knockdown phenotypes. This indicates that hnRNPK and Abi-1 act synergistic in a multiprotein complex that regulates the crucial balance between filopodia formation and synaptic maturation in neurons.

Gamete membrane microdomains and their associated molecules in fertilization signaling

Fertilization is the fundamental system of biological reproduction in many organisms, including animals, plants, and algae. A growing body of knowledge has emerged to explain how fertilization and activation of development are accomplished. Studies on the molecular mechanisms of fertilization are in progress for a wide variety of multicellular organisms. In this review, we summarize recent findings and debates about the long-standing questions concerning fertilization: how egg and sperm become competent for their interaction with each other, how the binding and fusion of these gamete cells are made possible, and how the fertilized eggs initiate development to a newborn. We will focus on the structure and function of the membrane microdomains (MDs) of egg and sperm that may serve as a platform or signaling center for the aforementioned cellular functions. In particular, we provide evidence that MDs of eggs from the African clawed frog, Xenopus laevis, play a pivotal role in receiving extracellular signals from fertilizing sperm and then transmitting them to the egg cytoplasm, where the tyrosine kinase Src is present and responsible for the subsequent signaling events collectively called egg activation. The presence of a new signaling axis involving uroplakin III, an MD-associated transmembrane protein, and Src in this system will be highlighted and discussed.

Hnrnpk, a protein differentially expressed in immature rat ovarian development, is required for normal primordial follicle assembly and development

The formation of ovarian follicles and subsequent development after birth are critical processes for female reproduction, and inappropriate coordination of these processes contributes to ovarian pathologies, such as premature ovarian failure and infertility. Identification and functional investigation of the factors involved in follicular assembly and the initial recruitment will be of great significance to the understanding of the female reproduction process. In this study, we examined the roles of transcription factor heterogeneous nuclear ribonucleoprotein K (Hnrnpk) in rat primordial folliculogenesis using RNA interference knockdown strategies. Reducing Hnrnpk mRNA levels via Hnrnpk small interfering RNAs to neonatal ovaries resulted in a substantial loss of naked oocytes, primordial and primary follicles. Structure disorganization of the ovary characterized by groups of oocytes arranged in nests, clusters of somatic cells not associated with any oocytes and many highly condensed oocyte nuclei was observed. Terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick end labeling assay demonstrated that these abnormalities may be partially attributable to abnormal apoptosis of oocytes. Furthermore, the microarray analysis showed that 63 genes changed significantly (≥2-folds or ≤0.5-fold) between the ovaries treated with Hnrnpk small interfering RNAs and the controls, with 22 up-regulated genes and 41 down-regulated genes. These differentially expressed genes were involved in several critical biological processes in ovarian development. These results suggest that transcription factor Hnrnpk is a key regulator for primordial follicle assembly and development, which provides a new potential therapeutic target to regulate ovarian function and treat ovarian disease.

Phosphoproteome study reveals Hsp27 as a novel signaling molecule involved in GDNF-induced neurite outgrowth

Glial-cell-line-derived neurotrophic factor (GDNF) is a most potent survival factor for dopaminergic neurons. In addition, GDNF was also found to promote neurite outgrowth in dopaminergic neurons. However, despite the potential clinical and physiological importance of GDNF, its mechanism of action is unclear. Therefore, we employed a state-of-the-art proteomic technique, DIGE (Difference in two-dimensional gel electrophoresis), to quantitatively compare profiles of phosphoproteins of PC12-GFRalpha1-RET cells (that stably overexpress GDNF receptor alpha1 and RET) 0.5 and 10 h after GDNF challenge with control. A total of 92 differentially expressed proteins were successfully identified by mass spectrometry. Among them, the relative levels of phosphorylated Hsp27 increased significantly both in 0.5 and 10 h GDNF-treated PC12-GFRalpha1-RET cells. Confocal microscopy and Western blot results showed that the phosphorylation of Hsp27 after GDNF treatment was accompanied by its nuclear translocation. After the mRNA of Hsp27 was interfered, neurite outgrowth of PC12-GFRalpha1-RET cells induced by GDNF was significantly blocked. Furthermore, the percentage of neurite outgrowth induced by GDNF was also reduced by the expression of dominant-negative mutants of Hsp27, in which specific serine phosphorylation residues (Ser15, Ser78 and Ser82) were substituted with alanine. Our data also revealed that p38 MAPK and ERK are the upstream regulators of Hsp27 phosphorylation. Hence, in addition to the numerous novel proteins that are potentially important in GDNF mediated differentiation of dopaminergic cells revealed by our study, our data has indicated that Hsp27 is a novel signaling molecule involved in GDNF-induced neurite outgrowth of dopaminergic neurons.

Vg1RBP phosphorylation by Erk2 MAP kinase correlates with the cortical release of Vg1 mRNA during meiotic maturation of Xenopus oocytes

Xenopus Vg1RBP is a member of the highly conserved IMP family of four KH-domain RNA binding proteins, with roles in RNA localization, translational control, RNA stability, and cell motility. Vg1RBP has been implicated in localizing Vg1 mRNAs to the vegetal cortex during oogenesis, in a process mediated by microtubules and microfilaments, and in migration of neural crest cells in embryos. Using c-mos morpholino, kinase inhibitors, and constitutely active recombinant kinases we show that Vg1RBP undergoes regulated phosphorylation by Erk2 MAPK during meiotic maturation, on a single residue, S402, located between the KH2 and KH3 domains. Phosphorylation temporally correlates with the release of Vg1 mRNA from its tight cortical association, assayed in lysates in physiological salt buffers, but does not affect RNA binding, nor self-association of Vg1RBP. U0126, a MAP kinase inhibitor, prevents Vg1RBP cortical release and Vg1 mRNA solubilization in meiotically maturing eggs, while injection of MKK6-DD, a constitutively activated MAP kinase kinase, promotes the release of both Vg1RBP and Vg1 mRNA from insoluble cortical structures. We propose that Erk2 MAP kinase phosphorylation of Vg1RBP regulates the protein:protein-mediated association of Vg1 mRNP with the cytoskeleton and/or ER. Since the MAP kinase site in Vg1RBP is conserved in several IMP homologs, this modification also has important implications for the regulation of IMP proteins in somatic cells.

A crucial role for hnRNP K in axon development in Xenopus laevis

We report that hnRNP K, an RNA-binding protein implicated in multiple aspects of post-transcriptional gene control, is essential for axon outgrowth in Xenopus. Its intracellular localization was found to be consistent with one of its known roles as an mRNA shuttling protein. In early embryos, it was primarily nuclear, whereas later it occupied both the nucleus and cytoplasm to varying degrees in different neuronal subtypes. Antisense hnRNP K morpholino oligonucleotides (MOs) microinjected into blastomeres suppressed hnRNP K expression from neural plate stages through to at least stage 40. Differentiating neural cells in these embryos expressed several markers for terminally differentiated neurons but failed to make axons. Rescue experiments and the use of two separate hnRNP K MOs were carried out to confirm that these effects were specifically caused by knockdown of hnRNP K expression. For insights into the involvement of hnRNP K in neuronal post-transcriptional gene control at the molecular level, we compared effects on expression of the medium neurofilament protein (NF-M), the RNA for which binds hnRNP K, with that of peripherin, another intermediate filament protein, the RNA for which does not bind hnRNP K. hnRNP K knockdown compromised NF-M mRNA nucleocytoplasmic export and translation, but had no effect on peripherin. Because eliminating NF-M from Xenopus axons attenuates, but does not abolish, their outgrowth, hnRNP K must target additional RNAs needed for axon development. Our study supports the idea that translation of at least a subset of RNAs involved in axon development is controlled by post-transcriptional regulatory modules that have hnRNP K as an essential element.