Initiation of translation and cellular localization of Theileria annulata casein kinase IIalpha: implication for its role in host cell transformation

How can a traffic light properly work if it is always green? The paradox of CK2 signaling

CK2 is a constitutively active protein kinase that assuring a constant level of phosphorylation to its numerous substrates supports many of the most important biological functions. Nevertheless, its activity has to be controlled and adjusted in order to cope with the varying needs of a cell, and several examples of a fine-tune regulation of its activity have been described. More importantly, aberrant regulation of this enzyme may have pathological consequences, e.g. in cancer, chronic inflammation, neurodegeneration, and viral infection. Our review aims at summarizing our current knowledge about CK2 regulation. In the first part, we have considered the most important stimuli shown to affect protein kinase CK2 activity/expression. In the second part, we focus on the molecular mechanisms by which CK2 can be regulated, discussing controversial aspects and future perspectives.

Fatal cases of Theileria annulata infection in calves in Portugal associated with neoplastic-like lymphoid cell proliferation

This study was carried out to investigate fifteen cases of acute lethal infection of calves (<or= 4 months of age) by the protozoan parasite Theileria (T.) annulata in the south of Portugal. Calves developed multifocal to coalescent nodular skin lesions, similar to multicentric malignant lymphoma. Infestation with ticks (genus Hyalomma) was intense. Theileria was seen in blood and lymph node smears, and T. annulata infection was confirmed by isolation of schizont-transformed cells and sequencing of hypervariable region 4 of the 18S rRNA gene. At necropsy, hemorrhagic nodules or nodules with a hemorrhagic halo were seen, particularly in the skin, subcutaneous tissue, skeletal and cardiac muscles, pharynx, trachea and intestinal serosa. Histologically, nodules were formed by large, round, lymphoblastoid neoplastic-like cells. Immunohistochemistry (IHC) identified these cells as mostly CD3 positive T lymphocytes and MAC387 positive macrophages. A marker for B lymphocytes (CD79alphacy) labeled very few cells. T. annulata infected cells in these nodules were also identified by IHC through the use of two monoclonal antibodies (1C7 and 1C12) which are diagnostic for the parasite. It was concluded that the pathological changes observed in the different organs and tissues were caused by proliferation of schizont-infected macrophages, which subsequently stimulate a severe uncontrolled proliferation of uninfected T lymphocytes.

Schizonts of Theileria annulata interact with the microtubuli network of their host cell via the membrane protein TaSP

Intracellular leukoproliferative Theileria are unique as eukaryotic organisms that transform the immune cells of their ruminant host. Theileria utilize the uncontrolled proliferation for rapid multiplication and distribution into host daughter cells. The parasite distribution into the daughter cells is accompanied by a tight association with the host cell mitotic apparatus. Since the molecular basis for this interaction is largely unknown, we investigated the possible involvement of the immunodominant Theileria annulata surface protein, TaSP, in the attachment of the parasite to host cell microtubule network. Confocal microscopic analyses showed co-localization of the TaSP protein with alpha-tubulin and reciprocal immuno-co-precipitation experiments demonstrated an association of TaSP with alpha-tubulin in vivo. In addition, the partially expressed predicted extracellular domain of TaSP co-localized with the mitotic spindle of dividing cells and was co-immunoprecipitated with alpha-tubulin in transiently transfected Cos-7 cells devoid of other T. annulata expressed proteins. Pull-down studies showed that there is a direct interaction between TaSP and polymerized microtubules. Analysis of the interaction of TaSP and host microtubulin during host cell mitosis indicated that TaSP co-localizes and interacts with the spindle poles, the mitotic spindle apparatus and the mid-body. Moreover, TaSP was demonstrated to be localized to the microtubule organizing center and to physically interact with gamma-tubulin. These data support the notion that the TaSP-microtubule interaction may be playing a potential role in parasite distribution into daughter host cells and give rise to the speculation that TaSP may be involved in regulation of microtubule assembly in the host cell.

Secretion of proteins into host cells by Apicomplexan parasites

The phylum Apicomplexa consists of a diverse group of obligate, intracellular parasites. The distinct evolutionary pressures on these protozoans as they have adapted to their respective niches have resulted in a variety of methods that they use to interact with and modify their hosts. One of these is the secretion and trafficking of parasite proteins into the host cell. We review this process for Theileria, Toxoplasma and Plasmodium. We also present what is known about the mechanisms by which parasite proteins are exported into the host cell, as well as information on their known and putative functions once they have reached their final destination.

Identification, molecular characterization and subcellular localization of a Theileria annulata parasite protein secreted into the host cell cytoplasm

Intracellular leucoproliferative Theileria are unique as eukaryotic organisms that transform the immune cells of their ruminant host. Theileria utilize the uncontrolled proliferation for rapid multiplication and distribution into host daughter cells. The equal distribution of the schizont into the daughter cells is thought to be accomplished by a tight association with the host cell mitotic apparatus. In this study, we describe a highly conserved novel 37 kD Theileria annulata protein (TaSE). TaSE was found to be localized inside the parasite, the parasite membrane and within the host cell cytoplasm. Moreover, it co-localized at distinct points with host cell microtubules, which was especially apparent during mitosis, where co-localization was found with the centromere, the mitotic spindle and the midbody. Association of TaSE with the host cell tubulin network was corroborated by coimmunoprecipitation and transient transfection experiments. This is the first description of a theilerial protein co-localizing and potentially interacting with a host cell protein. The distribution of TaSE during mitosis makes it a protein to consider as playing a potential role for parasite distribution into daughter host cells.

Constitutively activated CK2 potentially plays a pivotal role in Theileria-induced lymphocyte transformation

Activation of casein kinase II (CK2) was one of the first observations made on how Theileria parasites manipulate host cell signal transduction pathways and we argue that CK2 induction may in fact contribute to many of the different activation events that have been described since 1993 for Theileria-infected lymphocytes such as sustained activation of transcription factors c-Myc and NF-κB. CK2 also contributes to infected lymphocyte survival by inhibiting caspase activation and is probably behind constitutive PI3-K activation by phosphorylating PTEN. Finally, we also discuss how CK2A may act not only as a kinase, but also as a stimulatory subunit for the protein phosphatase PP2A, so dampening down the MEK/ERK and Akt/PKB pathways and for all these reasons we propose CK2 as a central player in Theileria-induced lymphocyte transformation.

Alteration of host cell phenotype by Theileria annulata and Theileria parva: mining for manipulators in the parasite genomes

The apicomplexan parasites Theileria annulata and Theileria parva cause severe lymphoproliferative disorders in cattle. Disease pathogenesis is linked to the ability of the parasite to transform the infected host cell (leukocyte) and induce uncontrolled proliferation. It is known that transformation involves parasite dependent perturbation of leukocyte signal transduction pathways that regulate apoptosis, division and gene expression, and there is evidence for the translocation of Theileria DNA binding proteins to the host cell nucleus. However, the parasite factors responsible for the inhibition of host cell apoptosis, or induction of host cell proliferation are unknown. The recent derivation of the complete genome sequence for both T. annulata and T. parva has provided a wealth of information that can be searched to identify molecules with the potential to subvert host cell regulatory pathways. This review summarizes current knowledge of the mechanisms used by Theileria parasites to transform the host cell, and highlights recent work that has mined the Theileria genomes to identify candidate manipulators of host cell phenotype.

Protein kinase casein kinase 2 holoenzyme produced ectopically in human cells can be exported to the external side of the cellular membrane

Ectokinases can phosphorylate extracellular proteins and external domains of membrane proteins influencing cell adhesion, movement, and cellular interactions. An ectokinase with the properties of casein kinase 2 (CK2) has been previously described, but little is known about the structural characteristics that allow this enzyme to be exported from the cell. Transfection of human embryonic kidney-293 cells with cDNAs coding for the catalytic (CK2alpha or CK2alpha') and regulatory (CK2beta) subunits with hemaglutinin tags allowed us to study the export of ectopically synthesized enzyme. When the catalytic (CK2alpha or CK2alpha') and the CK2beta regulatory subunits are cotransfected, the tetrameric enzyme composed of both subunits (holoenzyme) is detected outside the cell. This observation has been confirmed by assaying protein kinase activity in immunoprecipitates obtained with antihemaglutinin antibody by using a CK2-specific peptide substrate and by Western blots as well as by immunofluorescence of nonpermeabilized cells. Transfection with cDNA of catalytic or regulatory subunit alone does not result in export of these subunits. A study of the kinetics of appearance of the ectopically synthesized protein at different times after transfection indicates that a 5- to 7-h delay after the synthesis of the protein before it appears in the extracellular compartment. Using mutations of CK2alpha that eliminate phosphorylating activity [CK2alpha(Asp-156-Ala)] or that make it less sensitive to heparin inhibition [CK2alpha(Lys-75-Glu,Lys-76-Glu)] demonstrated that these mutations do not prevent the holoenzyme to be exported from the cells.