Protein phosphatase subunit G5PR that regulates the JNK-mediated apoptosis signal is essential for the survival of CD4 and CD8 double-positive thymocytes

Biased holoenzyme assembly of protein phosphatase 2A (PP2A): From cancer to small molecules

Protein phosphatase 2A (PP2A) is a family of serine threonine phosphatases responsible for regulating protein phosphorylation, thus opposing the activity of cellular kinases. PP2A is composed of a catalytic subunit (PP2A Cα/β) and scaffolding subunit (PP2A Aα/β) and various substrate-directing B regulatory subunits. PP2A biogenesis is regulated at multiple levels. For example, the sequestration of the free catalytic subunit during the process of biogenesis avoids promiscuous phosphatase activity. Posttranslational modifications of PP2A C direct PP2A heterotrimeric formation. Additionally, PP2A functions as a haploinsufficient tumor suppressor, where attenuated PP2A enzymatic activity creates a permissive environment for oncogenic transformation. Recent work studying PP2A in cancer showed that its role in tumorigenesis is more nuanced, with some holoenzymes being tumor suppressive, while others are required for oncogenic transformation. In cancer biology, PP2A function is modulated through various mechanisms including the displacement of specific B regulatory subunits by DNA tumor viral antigens, by recurrent mutations, and through loss of carboxymethyl-sensitive heterotrimeric complexes. In aggregate, these alterations bias PP2A activity away from its tumor suppressive functions and toward oncogenic ones. From a therapeutic perspective, molecular glues and disruptors present opportunities for both the selective stabilization of tumor-suppressive holoenzymes and disruption of holoenzymes that are pro-oncogenic. Collectively, these approaches represent an attractive cancer therapy for a wide range of tumor types. This review will discuss the mechanisms by which PP2A holoenzyme formation is dysregulated in cancer and the current therapies that are aimed at biasing heterotrimer formation of PP2A for the treatment of cancer.

Case Report: Novel Compound Heterozygotic Variants in PPP2R3C Gene Causing Syndromic 46, XY Gonadal Dysgenesis and Literature Review

Purpose: Patients with syndromic 46, XY disorders/differences of sex development (DSD) are characterized by gonadal and phenotypic genders inconsistent with their chromosomal sexes as well as abnormalities of multiple extragonadal organs. They are caused by mutations in specific genes, which are expressed in the affected organs and regulate their development, and over fourteen genes have been identified. In this study, we aimed to determine the underlying cause of a patient with syndromic 46, XY DSD and review the clinical presentations and genetic findings of all reported similar cases.

Methods: Whole-exome sequencing (WES) was performed to find a molecular cause of the patient. In silico tools were used to analyze the pathogenicity of the variants. Reports of cases with similar clinical features and involved genes were summarized by searching through PubMed/MEDLINE using keywords “PPP2R3C” or “G5PR” and “46,XY disorders of sex development”.

Results: Compound heterozygous variants (p.F229del/p.G417E) in PPP2R3C were identified in the 24-year-old female by WES and verified by Sanger sequencing. The patient presents complete testicular dysgenesis, low birth weight, facial deformity, cubitus valgus, and decreasing number of CD19⁺ B lymphocytes and CD4⁺ T lymphocytes. A total of thirteen 46, XY DSD cases with four homozygous PPP2R3C mutations (p.Leu103Pro, p.Leu193Ser, p.Phe350Ser, and p.Ser216_Tyr218dup) have been reported previously, and their clinical manifestations are roughly similar to those of our patient.

Conclusion: Novel compound heterozygous variants in PPP2R3C cause specific syndromic 46, XY gonadal dysgenesis, which broadened the pathogenic variants spectrum of PPP2R3C. The typical phenotype of PPP2R3C mutation is complete 46, XY gonadal dysgenesis with multiple extragonadal anomalies, including facial deformities, skeletal system abnormalities, muscle abnormalities, impaired nervous system, impaired hearing and vision, heart and kidney anomalies, and gastrointestinal dysfunction.

Expanding the spectrum of syndromic PPP2R3C-related XY gonadal dysgenesis to XX gonadal dysgenesis

Homozygous variants in PPP2R3C have been reported to cause a syndromic 46,XY complete gonadal dysgenesis phenotype with extragonadal manifestations (GDRM, MIM# 618419) in patients from four unrelated families, whereas heterozygous variants have been linked to reduced fertility with teratozoospermia (SPGF36, MIM# 618420) in male carriers. We present eight patients from four unrelated families of Turkish and Indian descent with three different germline homozygous PPP2R3C variants including a novel in-frame duplication (c.639_647dupTTTCTACTC, p.Ser216_Tyr218dup). All patients exhibit recognizable facial dysmorphisms allowing gestalt diagnosis. In two 46,XX patients with hypergonadotropic hypogonadism and nonvisualized gonads, primary amenorrhea along with absence of secondary sexual characteristics and/or unique facial gestalt led to the diagnosis. 46,XY affected individuals displayed a spectrum of external genital phenotypes from ambiguous genitalia to complete female. We expand the spectrum of syndromic PPP2R3C-related XY gonadal dysgenesis to both XY and XX gonadal dysgenesis. Our findings supported neither ocular nor muscular involvement as major criteria of the syndrome. We also did not encounter infertility problems in the carriers. Since both XX and XY individuals were affected, we hypothesize that PPP2R3C is essential in the early signaling cascades controlling sex determination in humans.

Germinal Center B-Cell-Associated Nuclear Protein (GANP) Involved in RNA Metabolism for B Cell Maturation

Germinal center B-cell-associated nuclear protein (GANP) is upregulated in germinal center B cells against T-cell-dependent antigens in mice and humans. In mice, GANP depletion in B cells impairs antibody affinity maturation. Conversely, its transgenic overexpression augments the generation of high-affinity antigen-specific B cells. GANP associates with AID in the cytoplasm, shepherds AID into the nucleus, and augments its access to the rearranged immunoglobulin (Ig) variable (V) region of the genome in B cells, thereby precipitating the somatic hypermutation of V region genes. GANP is also upregulated in human CD4(+) T cells and is associated with APOBEC3G (A3G). GANP interacts with A3G and escorts it to the virion cores to potentiate its antiretroviral activity by inactivating HIV-1 genomic cDNA. Thus, GANP is characterized as a cofactor associated with AID/APOBEC cytidine deaminase family molecules in generating diversity of the IgV region of the genome and genetic alterations of exogenously introduced viral targets. GANP, encoded by human chromosome 21, as well as its mouse equivalent on chromosome 10, contains a region homologous to Saccharomyces Sac3 that was characterized as a component of the transcription/export 2 (TREX-2) complex and was predicted to be involved in RNA export and metabolism in mammalian cells. The metabolism of RNA during its maturation, from the transcription site at the chromosome within the nucleus to the cytoplasmic translation apparatus, needs to be elaborated with regard to acquired and innate immunity. In this review, we summarize the current knowledge on GANP as a component of TREX-2 in mammalian cells.

Isolation, identification, and purification of murine thymic epithelial cells

The thymus is a vital organ for T lymphocyte development. Of thymic stromal cells, thymic epithelial cells (TECs) are particularly crucial at multiple stages of T cell development: T cell commitment, positive selection and negative selection. However, the function of TECs in the thymus remains incompletely understood. In the article, we provide a method to isolate TEC subsets from fresh mouse thymus using a combination of mechanical disruption and enzymatic digestion. The method allows thymic stromal cells and thymocytes to be efficiently released from cell-cell and cell-extracellular matrix connections and to form a single-cell suspension. Using the isolated cells, multiparameter flow cytometry can be applied to identification and characterization of TECs and dendritic cells. Because TECs are a rare cell population in the thymus, we also describe an effective way to enrich and purify TECs by depleting thymocytes, the most abundant cell type in the thymus. Following the enrichment, cell sorting time can be decreased so that loss of cell viability can be minimized during purification of TECs. Purified cells are suitable for various downstream analyses like Real Time-PCR, Western blot and gene expression profiling. The protocol will promote research of TEC function and as well as the development of in vitro T cell reconstitution.

Increased expression of the pro-apoptotic protein BIM, a mechanism for cAMP/protein kinase A (PKA)-induced apoptosis of immature T cells

The second messenger cAMP is proapoptotic for numerous cell types, but the mechanism for this proapoptotic action is not defined. Here, we use murine CD4(+)/CD8(+) S49 lymphoma cells and isolated thymocytes to assess this mechanism. In WT S49 cells, cAMP acts via protein kinase A (PKA) to induce G(1) phase cell cycle arrest and apoptosis. Treatment of WT and cAMP-Deathless (D-) S49 cells, which lack cAMP-promoted apoptosis, with the PKA agonist 8-(4-chlorophenylthio)-cAMP (CPT-cAMP) differentially regulates transcripts for numerous proapoptotic and antiapoptotic proteins. In contrast, kin-S49 cells (which lack PKA) show no cAMP-promoted changes in transcript expression. In this study, we use knockdown and overexpression approaches to define the role in cAMP/PKA-promoted apoptosis of the proapoptotic factor BIM (Bcl-2 interacting mediator of cell death), whose expression prominently increases in response to CPT-cAMP treatment of WT but not D- or kin- S49 cells. Conditional expression of BimL, one of the three major forms of Bim, increases apoptosis of WT, D-, and kin-S49 cells, whereas inhibition of cAMP-mediated induction of Bim isoforms by shRNAi attenuates CPT-cAMP-mediated apoptosis of WT S49 cells. Bim protein levels increase in subpopulations of CPT-cAMP-treated cells that undergo apoptosis. Thymic CD4(+)/CD8(+) cells isolated from Bim(-/-) mice corroborated the requirement of Bim expression for cAMP-promoted apoptosis. Thus, up-regulation of Bim appears to be an important determinant of cAMP/PKA-mediated apoptosis in immature T cells and may be a mechanism for such apoptosis in other cell types as well.

Phosphorylation-dependent association of the G4-1/G5PR regulatory subunit with IKKβ negatively modulates NF-κB activation through recruitment of protein phosphatase 5

The transcription factor NF-κB (nuclear factor κB) co-ordinates various gene expressions in response to diverse signals and is a critical regulator of inflammation and innate immunity. Several negative regulators of NF-κB have been identified as downstream targets of NF-κB and function as a feedback control of NF-κB activation. A few protein phosphatases have also been shown to inactivate NF-κB activation. However, little is known about how protein phosphatases detect and respond to NF-κB activation. In the present study, we report a regulatory subunit of PP5 (protein phosphatase 5), G4-1, that physically interacts with IKKβ [IκB (inhibitor of NF-κB) kinase β] and negatively regulates NF-κB activation. The association of G4-1 with IKKβ depends on the kinase activity of IKKβ. Mapping of the G4-1-binding domain of IKKβ reveals that the serine-rich domain in the C-terminus of IKKβ is required for G4-1 binding. When seven autophosphorylated serine residues in this domain were mutated to alanine, the mutant form of IKKβ lost its ability to bind G4-1 and was more potent than the wild-type kinase to activate NF-κB. Knockdown of G4-1 enhanced TNFα (tumour necrosis factor α)-induced NF-κB activity, and knockdown of PP5 totally abolished the inhibitory activity of G4-1 on NF-κB activation. The results of the present study suggest that G4-1 functions as an adaptor to recruit PP5 to the phosphorylated C-terminus of activated IKKβ and to down-regulate the activation of IKKβ.

Central tolerance: what have we learned from mice?

Producing a healthy immune system capable of defending against pathogens, while avoiding autoimmunity, is dependent on thymic selection. Positive selection yields functional T cells that have the potential to recognize both self and foreign antigens. Therefore, negative selection exists to manage potentially self-reactive cells. Negative selection results from the induction of anergy, receptor editing, clonal diversion (agonist selection), and/or clonal deletion (apoptosis) in self-reactive clones. Clonal deletion has been inherently difficult to study because the cells of interest are undergoing apoptosis and being eliminated quickly. Furthermore, analysis of clonal deletion in humans has proved even more difficult due to availability of samples and lack of reagents. Mouse models have thus been instrumental in achieving our current understanding of central tolerance, and the evolution of elegant model systems has led to an explosion of new data to be assimilated. This review will focus on recent advances in the field of clonal deletion with respect to three aspects: the development of physiological model systems, signaling pathways that lead to apoptosis, and antigen presenting cell types involved in the induction of clonal deletion.

Diversity in genomic organisation, developmental regulation and distribution of the murine PR72/B" subunits of protein phosphatase 2A

Background

Protein phosphatase 2A (PP2A) is a serine/threonine-specific phosphatase displaying vital functions in growth and development through its role in various signalling pathways. PP2A holoenzymes comprise a core dimer composed of a catalytic C and a structural A subunit, which can associate with a variable B-type subunit. The importance of the B-type subunits for PP2A regulation cannot be overestimated as they determine holoenzyme localisation, activity and substrate specificity. Three B-type subunit families have been identified: PR55/B, PR61/B' and PR72/B", of which the latter is currently the least characterised.

Results

We deduced the sequences and genomic organisation of the different murine PR72/B" isoforms: three genes encode nine isoforms, five of which are abundantly expressed and give rise to genuine PP2A subunits. Thereby, one novel subunit was identified. Using Northern blotting, we examined the tissue-specific and developmental expression of these subunits. All subunits are highly expressed in heart, suggesting an important cardiac function. Immunohistochemical analysis revealed a striated expression pattern of PR72 and PR130 in heart and skeletal muscle, but not in bladder smooth muscle. The subcellular localisation and cell cycle regulatory ability of several PR72/B" isoforms were determined, demonstrating differences as well as similarities.

Conclusion

In contrast to PR55/B and PR61/B', the PR72/B" family seems evolutionary more divergent, as only two of the murine genes have a human orthologue. We have integrated these results in a more consistent nomenclature of both human and murine PR72/B" genes and their transcripts/proteins. Our results provide a platform for the future generation of PR72/B" knockout mice.

Cytoplasmic domains of the transporter associated with antigen processing and P-glycoprotein interact with subunits of the proteasome

The proteasome is a multi-protein complex that degrades cellular proteins as well as foreign proteins destined for antigen presentation. The latter function involves the immunoproteasome, in which several proteasome subunits are exchanged for gamma-interferon-induced subunits. The transporter associated with antigen processing (TAP) transports proteasome-generated peptides across the membrane of the endoplasmic reticulum (ER) prior to presentation on the plasma membrane. We demonstrate interactions between the cytoplasmic domains of TAP subunits and subunits of both the proteasome and the immunoproteasome, suggesting direct targeting of antigenic peptides to the ER via a TAP-proteasome association. We also show interaction between one of the cytoplasmic domains of P-glycoprotein and a proteasome subunit, but not the corresponding immunoproteasome subunit, suggesting a possible role for P-glycoprotein in the transport of proteasome-derived peptides.