Laure Twyffels

Shuttling SR proteins: more than splicing factors.

Serine–arginine (SR) proteins commonly designate a family of eukaryotic RNA binding proteins containing a protein domain composed of several repeats of the arginine–serine dipeptide, termed the arginine–serine (RS) domain. This protein family is involved in essential nuclear processes such as constitutive and alternative splicing of mRNA precursors. Besides participating in crucial activities in the nuclear compartment, several SR proteins are able to shuttle between the nucleus and the cytoplasm and to exert regulatory functions in the latter compartment. This review aims at discussing the properties of shuttling SR proteins with particular emphasis on their nucleo‐cytoplasmic traffic and their cytoplasmic functions. Indeed, recent findings have unravelled the complex regulation of SR protein nucleo‐cytoplasmic distribution and the diversity of cytoplasmic mechanisms in which these proteins are involved.

Transportin-1 and Transportin-2: protein nuclear import and beyond.

Nearly 20 years after its identification as a new β‐karyopherin mediating the nuclear import of the RNA‐binding protein hnRNP A1, Transportin‐1 is still commonly overlooked in comparison with its best known cousin, Importin‐β. Transportin‐1 is nonetheless a considerable player in nucleo‐cytoplasmic transport. Over the past few years, significant progress has been made in the characterization of the nuclear localization signals (NLSs) that Transportin‐1 recognizes, thereby providing the molecular basis of its diversified repertoire of cargoes. The recent discovery that mutations in the Transportin‐dependent NLS of FUS cause mislocalization of this protein and result in amyotrophic lateral sclerosis illustrates the importance of Transportin‐dependent import for human health. Besides, new functions of Transportin‐1 are emerging in processes other than nuclear import. Here, we summarize what is known about Transportin‐1 and the related β‐karyopherin Transportin‐2.

Anoctamin-1/TMEM16A is the major apical iodide channel of the thyrocyte.

Iodide is captured by thyrocytes through the Na(+)/I(-) symporter (NIS) before being released into the follicular lumen, where it is oxidized and incorporated into thyroglobulin for the production of thyroid hormones. Several reports point to pendrin as a candidate protein for iodide export from thyroid cells into the follicular lumen. Here, we show that a recently discovered Ca(2+)-activated anion channel, TMEM16A or anoctamin-1 (ANO1), also exports iodide from rat thyroid cell lines and from HEK 293T cells expressing human NIS and ANO1. The Ano1 mRNA is expressed in PCCl3 and FRTL-5 rat thyroid cell lines, and this expression is stimulated by thyrotropin (TSH) in rat in vivo, leading to the accumulation of the ANO1 protein at the apical membrane of thyroid follicles. Moreover, ANO1 properties, i.e., activation by intracellular calcium (i.e., by ionomycin or by ATP), low but positive affinity for pertechnetate, and nonrequirement for chloride, better fit with the iodide release characteristics of PCCl3 and FRTL-5 rat thyroid cell lines than the dissimilar properties of pendrin. Most importantly, iodide release by PCCl3 and FRTL-5 cells is efficiently blocked by T16Ainh-A01, an ANO1-specific inhibitor, and upon ANO1 knockdown by RNA interference. Finally, we show that the T16Ainh-A01 inhibitor efficiently blocks ATP-induced iodide efflux from in vitro-cultured human thyrocytes. In conclusion, our data strongly suggest that ANO1 is responsible for most of the iodide efflux across the apical membrane of thyroid cells.

Neonatal Follicular Th Cell Responses Are Impaired and Modulated by IL-4

Newborns are characterized by poor responses to vaccines. Defective B cell responses and a Th2-type polarization can account for this impaired protection in early life. We in this study investigated the generation of follicular Th (TFH) cells, involved in the development of Ab response and germinal center reaction, upon vaccination in neonates. We showed that, compared with adults, Ab production, affinity maturation, and germinal center formation were reduced in neonates immunized with OVA–aluminum hydroxide. Although this vaccination induced CD4+ CXCR5+ PD-1+ TFH cells in newborns, their frequency, as well as their Bcl6 expression and IL-21 and IL-4 mRNA induction, was decreased in early life. Moreover, neonatal TFH cells were mainly localized in interfollicular regions of lymphoid tissues. The prototypic Th2 cytokine IL-4 was found to promote the emergence and the localization in germinal centers of neonatal TFH cells, as well as the neonatal germinal center reaction itself. In addition, IL-4 dampened expression of Th17-related molecules in neonatal TFH cells, as TFH cells from immunized IL-4–deficient neonates displayed enhanced expression of RORγt and IL-17. This Th17-like profile correlated with an increased secretion of OVA-specific IgG2a. Our study thus suggests that defective humoral immunity in early life is associated with limited and IL-4–modulated TFH cell responses.

Pendrin: the Thyrocyte Apical Membrane Iodide Transporter?

In the thyroid, the transport of iodide from the extracellular space to the follicular lumen requires two steps: the transport in the cell at the basal side and in the lumen at the apical side. The first step is mediated by the Na+/I- symporter (NIS). In most reviews and textbooks, the second step is presented as mediated by pendrin. In this review, we analyze this assumption. There are several arguments supporting the concept that indeed pendrin plays an important role in thyroid physiology. However, biochemical, clinical and histological data on the thyroid of a patient with Pendred syndrome do not suggest an essential role in iodide transport, which is corroborated by the lack of a thyroid phenotype in pendrin knockout mice. Experiments in vivo and in vitro on polarized and unpolarized cells show that iodide is transported transport of iodide at the apex of the thyroid cell. Moreover, ectopic expression of pendrin in transfected non-thyroid cells is capable of mediating iodide efflux. It is concluded that pendrin may participate in the iodide efflux into thyroid lumen but not as the unique transporter. Moreover, another role of pendrin in mediating Cl-/HCO3- exchange and controlling luminal pH is suggested.

Post-transcriptional Regulation of Genes Encoding Anti-microbial Peptides in Drosophila

Secretion of antimicrobial peptides (AMPep) is a central defense mechanism used by invertebrates to combat infections. In Drosophila the synthesis of these peptides is a highly regulated process allowing their rapid release in the hemolymph upon contact with pathogens and the arrest of their production after pathogen clearance. We observed that AMPep genes have either a transient or sustained expression profile in S2 Drosophila cells treated with peptidoglycan. Moreover, AMPep genes containing AU-rich elements (ARE) in their 3′-untranslated region (UTR) are subject to a post-transcriptional control affecting mRNA stability, thereby contributing to their transient expression profile. Cecropin A1 (CecA1) constitutes the prototype of this latter class of AMPeps. CecA1 mRNA bears in its 3′-UTR an ARE similar to class II AREs found in several short-lived mammalian mRNAs. In response to immune deficiency cascade signaling activated by Gram-negative peptidoglycans, CecA1 mRNA is transiently stabilized and subsequently submitted to deadenylation and decay mediated by the ARE present in its 3′-UTR. The functionality of CecA1 ARE relies on its ability to recruit TIS11 protein, which accelerates CecA1 mRNA deadenylation and decay. Moreover, we observed that CecA1 mRNA deadenylation is a biphasic process. Whereas early deadenylation is independent of TIS11, the later deadenylation phase depends on TIS11 and is mediated by CAF1 deadenylase. We also report that in contrast to tristetraprolin, its mammalian homolog, TIS11, is constitutively expressed in S2 cells and accumulates in cytoplasmic foci distinct from processing bodies, suggesting that the Drosophila ARE-mediated mRNA deadenylation and decay mechanism is markedly different in invertebrates and mammals.

Rapid Proteasomal Degradation of Posttranscriptional Regulators of the TIS11/Tristetraprolin Family Is Induced by an Intrinsically Unstructured Region Independently of Ubiquitination

ABSTRACT The TIS11/tristetraprolin (TTP) CCCH tandem zinc finger proteins are major effectors in the destabilization of mRNAs bearing AU-rich elements (ARE) in their 3′ untranslated regions. In this report, we demonstrate that the Drosophila melanogaster dTIS11 protein is short-lived due to its rapid ubiquitin-independent degradation by the proteasome. Our data indicate that this mechanism is tightly associated with the intrinsically unstructured, disordered N- and C-terminal domains of the protein. Furthermore, we show that TTP, the mammalian TIS11/TTP protein prototype, shares the same three-dimensional characteristics and is degraded by the same proteolytic pathway as dTIS11, thereby indicating that this mechanism has been conserved across evolution. Finally, we observed a phosphorylation-dependent inhibition of dTIS11 and TTP degradation by the proteasome in vitro, raising the possibility that such modifications directly affect proteasomal recognition for these proteins. As a group, RNA-binding proteins (RNA-BPs) have been described as enriched in intrinsically disordered regions, thus raising the possibility that the mechanism that we uncovered for TIS11/TTP turnover is widespread among other RNA-BPs.

A Masked PY-NLS in Drosophila TIS11 and Its Mammalian Homolog Tristetraprolin.

Many RNA-binding proteins (RBPs) dynamically shuttle between the nucleus and the cytoplasm, often exerting different functions in each compartment. Therefore, the nucleo-cytoplasmic distribution of RBPs has a strong impact on their activity. Here we describe the localization and the shuttling properties of the tandem zinc finger RBP dTIS11, which is the Drosophila homolog of mammalian TIS11 proteins. Drosophila and mammalian TIS11 proteins act as destabilizing factors in ARE-mediated decay. At equilibrium, dTIS11 is concentrated mainly in the cytoplasm. We show that dTIS11 is a nucleo-cytoplasmic shuttling protein whose nuclear export is mediated by the exportin CRM1 through the recognition of a nuclear export signal (NES) located in a different region comparatively to its mammalian homologs. We also identify a cryptic Transportin-dependent PY nuclear localization signal (PY-NLS) in the tandem zinc finger region of dTIS11 and show that it is conserved across the TIS11 protein family. This NLS partially overlaps the second zinc finger ZnF2. Importantly, mutations disrupting the capacity of the ZnF2 to coordinate a Zinc ion unmask dTIS11 and TTP NLS and promote nuclear import. All together, our results indicate that the nuclear export of TIS11 proteins is mediated by CRM1 through diverging NESs, while their nuclear import mechanism may rely on a highly conserved PY-NLS whose activity is negatively regulated by ZnF2 folding.

CDK4 phosphorylation status and a linked gene expression profile predict sensitivity to palbociclib

Cyclin D‐CDK4/6 are the first CDK complexes to be activated in the G1 phase in response to oncogenic pathways. The specific CDK4/6 inhibitor PD0332991 (palbociclib) was recently approved by the FDA and EMA for treatment of advanced ER‐positive breast tumors. Unfortunately, no reliable predictive tools are available for identifying potentially responsive or insensitive tumors. We had shown that the activating T172 phosphorylation of CDK4 is the central rate‐limiting event that initiates the cell cycle decision and signals the presence of active CDK4. Here, we report that the profile of post‐translational modification including T172 phosphorylation of CDK4 differs among breast tumors and associates with their subtypes and risk. A gene expression signature faithfully predicted CDK4 modification profiles in tumors and cell lines. Moreover, in breast cancer cell lines, the CDK4 T172 phosphorylation best correlated with sensitivity to PD0332991. This gene expression signature identifies tumors that are unlikely to respond to CDK4/6 inhibitors and could help to select a subset of patients with HER2‐positive and basal‐like tumors for clinical studies on this class of drugs.

Dual RNA Processing Roles of Pat1b via Cytoplasmic Lsm1-7 and Nuclear Lsm2-8 Complexes

Summary Pat1 RNA-binding proteins, enriched in processing bodies (P bodies), are key players in cytoplasmic 5′ to 3′ mRNA decay, activating decapping of mRNA in complex with the Lsm1-7 heptamer. Using co-immunoprecipitation and immunofluorescence approaches coupled with RNAi, we provide evidence for a nuclear complex of Pat1b with the Lsm2-8 heptamer, which binds to the spliceosomal U6 small nuclear RNA (snRNA). Furthermore, we establish the set of interactions connecting Pat1b/Lsm2-8/U6 snRNA/SART3 and additional U4/U6.U5 tri-small nuclear ribonucleoprotein particle (tri-snRNP) components in Cajal bodies, the site of snRNP biogenesis. RNA sequencing following Pat1b depletion revealed the preferential upregulation of mRNAs normally found in P bodies and enriched in 3′ UTR AU-rich elements. Changes in >180 alternative splicing events were also observed, characterized by skipping of regulated exons with weak donor sites. Our data demonstrate the dual role of a decapping enhancer in pre-mRNA processing as well as in mRNA decay via distinct nuclear and cytoplasmic Lsm complexes.