MNDA dimerizes through a complex motif involving an N-terminal basic region

Activation and Immune Regulation Mechanisms of PYHIN Family During Microbial Infection

The innate immune system defenses against pathogen infections via patten-recognition receptors (PRRs). PRRs initiate immune responses by recognizing pathogen-associated molecular patterns (PAMPs), including peptidoglycan, lipopolysaccharide, and nucleic acids. Several nucleic acid sensors or families have been identified, such as RIG-I-like receptors (RLRs), Toll-like receptors (TLRs), cyclic GMP-AMP synthase (cGAS), and PYHIN family receptors. In recent years, the PYHIN family cytosolic DNA receptors have increased attention because of their important roles in initiating innate immune responses. The family members in humans include Absent in melanoma 2 (AIM2), IFN-γ inducible protein 16 (IFI16), interferon-inducible protein X (IFIX), and myeloid cell nuclear differentiation antigen (MNDA). The PYHIN family members are also identified in mice, including AIM2, p202, p203, p204, and p205. Herein, we summarize recent advances in understanding the activation and immune regulation mechanisms of the PYHIN family during microbial infection. Furthermore, structural characterizations of AIM2, IFI16, p202, and p204 provide more accurate insights into the signaling mechanisms of PYHIN family receptors. Overall, the molecular details will facilitate the development of reagents to defense against viral infections.

Myeloid cell nuclear differentiation antigen controls the pathogen-stimulated type I interferon cascade in human monocytes by transcriptional regulation of IRF7

Type I interferons (IFNs) are critical for anti-viral responses, and also drive autoimmunity when dysregulated. Upon viral sensing, monocytes elicit a sequential cascade of IFNβ and IFNα production involving feedback amplification, but how exactly this cascade is regulated in human cells is incompletely understood. Here we show that the PYHIN protein myeloid cell nuclear differentiation antigen (MNDA) is required for IFNα induction in monocytes. Unlike other PYHINs, this is not due to a pathogen sensing role, but rather MNDA regulated expression of IRF7, a transcription factor essential for IFNα induction. Mechanistically, MNDA is required for recruitment of STAT2 and RNA polymerase II to the IRF7 gene promoter, and in fact MNDA is itself recruited to the IRF7 promoter after type I IFN stimulation. These data implicate MNDA as a critical regulator of the type I IFN cascade in human myeloid cells and reveal a new role for human PYHINs in innate immune gene induction.

The interferon response is a critical component of the innate immune response. Here the authors implicate MNDA in the regulation of type I interferon responses to pathogen infection.

PYHIN Proteins and HPV: Role in the Pathogenesis of Head and Neck Squamous Cell Carcinoma

In the last decades, the human papillomavirus (HPV) emerged as an etiological cause of head and neck squamous cell carcinoma (HNSCC), especially in the oropharynx. The role of two intracellular DNA sensors, which belong to the PYHIN family (interferon-inducible protein 16 (IFI16) and absent in melanoma 2 protein (AIM2)), has been analyzed in relation to HPV infection and head and neck carcinogenesis. In particular, IFI16 and AIM2 expression depends on HPV infection in HNSCC. They represent viral restriction factors and are key components of the intrinsic immunity activated against different viruses, including HPV. This review analyzed and summarized the recent findings about the role of PYHIN proteins in HPV⁺ and HPV⁻ HNSCC.

The emerging role of human PYHIN proteins in innate immunity: implications for health and disease

The innate immune response depends on the ability of immune cells to detect pathogens through germline-encoded pattern recognition receptors (PRRs). Recently discovered PRRs include some members of the Pyrin and HIN domain (PYHIN) family, which are encoded on an interferon-inducible gene cluster located on chromosome 1q23. There are five human PYHIN proteins; Absent in melanoma 2 (AIM2), IFN-γ inducible protein 16 (IFI16), Myeloid cell nuclear differentiation antigen (MNDA), Pyrin and HIN domain family member 1 (PYHIN1) and the recently identified Pyrin domain only protein 3 (POP3). Early studies reported roles for these proteins in cell cycle control, tumour suppression and transcriptional regulation. AIM2 and IFI16 have now been shown to be immune sensors of non-self DNA, such as that produced by viruses in infected cells. AIM2 binds DNA to activate the inflammasome, while IFI16 detection of DNA can lead to the up-regulation of type I interferons or inflammasome activation. Recent studies have shown how IFI16 senses DNA viruses, and also how viruses evade detection by IFI16, while structural studies have greatly advanced our understanding of how AIM2 and IFI16 bind DNA to activate these immune responses. Furthermore, following the identification of POP3, interplay between members of this gene cluster has been established, with POP3 acting as a negative regulator of the AIM2 and IFI16 inflammasomes. In this review we discuss the current understanding of how PYHIN proteins function in innate immunity, their role in disease and the therapeutic possibilities that arise as a result.

Myeloid nuclear differentiation antigen, neutrophil apoptosis and sepsis

Sepsis and septic shock are characterized by prolonged inflammation and delayed resolution, which are associated with suppression of neutrophil apoptosis. The role of the intrinsic apoptotic pathway and intracellular factors in regulation of neutrophil apoptosis remain incompletely understood. We previously reported that the nuclear factor MNDA (myeloid nuclear differentiation antigen) is fundamental to execution of the constitutive neutrophil death program. During neutrophil apoptosis MNDA is cleaved by caspases and relocated to the cytoplasm. However, when challenged with known mediators of sepsis, human neutrophils of healthy donors or neutrophils from patients with sepsis exhibited impaired MNDA relocation/cleavage parallel with myeloid cell leukemia-1 (MCL-1) accumulation and suppression of apoptosis. MNDA knockdown in a model cell line indicated that upon induction of apoptosis, MNDA promotes proteasomal degradation of MCL-1, thereby aggravating mitochondrial dysfunction. Thus, MNDA is central to a novel nucleus-mitochondrion circuit that promotes progression of apoptosis. Disruption of this circuit contributes to neutrophil longevity, thereby identifying MNDA as a potential therapeutic target in sepsis and other inflammatory pathologies.

Role for myeloid nuclear differentiation antigen in the regulation of neutrophil apoptosis during sepsis

RATIONALE

Suppressed neutrophil apoptosis, a hallmark of sepsis, perpetuates inflammation and delays resolution. Myeloid nuclear differentiation antigen (MNDA) is expressed only in myeloid cells and has been implicated in cell differentiation; however, its function in mature neutrophils is not known.

OBJECTIVES

We studied whether MNDA could contribute to regulation of apoptosis of neutrophils from healthy subjects and patients with sepsis, and investigated the impact of MNDA knockdown on apoptosis.

METHODS

Human neutrophils were challenged with mediators of sepsis and neutrophils from patients with sepsis were cultured to investigate cleavage and cytoplasmic accumulation of MNDA. MNDA was knocked down in myeloid HL-60 cells to investigate development of apoptosis.

MEASUREMENTS AND MAIN RESULTS

During constitutive apoptosis of human neutrophils, MNDA is cleaved by caspases and accumulated in the cytoplasm, where it promotes degradation of the antiapoptotic protein Mcl-1, thereby accelerating collapse of mitochondrial transmembrane potential. Culture of neutrophils with LPS, bacterial DNA, or platelet-activating factor prevented MNDA cleavage and cytoplasmic accumulation. MNDA knockdown with short hairpin RNA markedly attenuated Mcl-1 turnover and conferred resistance to stress-induced apoptosis in HL-60 cells. Neutrophils from patients with severe sepsis exhibited markedly suppressed apoptosis that was associated with impaired cytoplasmic MNDA accumulation, preservation of Mcl-1 expression, and mitochondrial transmembrane potential. Culture of neutrophils of healthy subjects with septic plasma delayed apoptosis and cytoplasmic MNDA accumulation.

CONCLUSIONS

These results indicate that cytoplasmic accumulation of MNDA facilitates progression of apoptosis and suggest that impaired cytoplasmic MNDA accumulation contributes to delayed neutrophil apoptosis in patients with severe sepsis.

Cloning and characterisation of Ifi206: a new murine HIN-200 family member

HIN-200 proteins are interferon-inducible proteins capable of regulating cell growth, senescence, differentiation and death. Using a combination of in silico analysis of NCBI EST databases and screening of murine C57BL/6 cDNA libraries we isolated novel murine HIN-200 cDNAs designated Ifi206S and Ifi206L encoding two putative mRNA splice variants. The p206S and p206L protein isoforms have a modular domain structure consisting of an N-terminal PAAD/DAPIN/Pyrin domain, a region rich in serine, threonine and proline residues and a C-terminal 200 B domain characteristic of other HIN-200 proteins. Ifi206 mRNA was detected only in the spleen and lung of BALB/c and C57BL/6 mice and expression was up-regulated by both types I and II IFN subtypes. p206 protein was predominantly expressed in the cytoplasm and addition of LMB, a CRM1 dependent nuclear export inhibitor, caused p206 to accumulate in the nucleus. Unlike other human and mouse HIN-200 proteins that contain only a single 200 amino acid domain, overexpression of p206 impaired the clonogenic growth of tumour cell lines. Thus, p206 represents the newest HIN-200 family member discovered. It has distinct and restricted pattern of expression however maintains many of the hallmarks of HIN-200 proteins including the presence of a characteristic 200 X domain, induction by interferon and an ability to suppress tumour cell growth.

Biochemical and growth regulatory activities of the HIN-200 family member and putative tumor suppressor protein, AIM2

The human HIN-200 family member AIM2 was originally identified in a screen for suppressors of melanoma tumorigenicity following introduction of chromosome 6 into the UACC903 human melanoma cell line. Although the AIM2 protein contained many of the conserved structural motifs common to other HIN-200 proteins, the biochemical characteristics of AIM2 and the ability of overexpressed AIM2 to phenocopy the effect of introduction of chromosome 6 in the UACC903 cells had not been assessed. Herein we demonstrated that AIM2 was localised within the nucleus of transfected or interferon-treated human cells. In addition, AIM2 could homodimerise via the amino-terminal (PAAD/DAPIN) region and heterodimerise with the related IFI 16 protein. However, overexpressed AIM2 did not significantly affect the growth or survival of UACC903 cells or another human melanoma cell line. These data indicate that AIM2 has many of the biochemical and structural characteristics of HIN-200 proteins, however, its expression is not sufficient to induce a tumor-suppressor-like phenotype.

Variable expression of human myeloid specific nuclear antigen MNDA in monocyte lineage cells in atherosclerosis

MNDA (human myeloid nuclear differentiation antigen) is expressed in specific lineages of hematopoietic cells and most notably at high levels in macrophages at sites of inflammation. MNDA and related proteins appear to modulate the activity of transcription factors and in some cases have a role in mediating cell death. The expression of MNDA was characterized in normal and diseased human aorta. MNDA positive cells double labeled for CD68 in all tissue examined. Twenty percent of normal aortas were negative or contained rare MNDA positive cells while other normal aorta contained more frequent positive cells. In atherosclerotic aorta, the number of MNDA positive cells increased with progression of disease. In normal and early lesions, MNDA positive cells adjacent to the endothelium generally displayed a strong MNDA reactivity associated with small amount of CD68 reactive cytoplasm. In the same sections, MNDA positive cells at increasing distances from the endothelium displayed lower MNDA reactivity and were associated with larger amounts of CD68 reactive cytoplasm. Foam cells in fatty streaks exhibited MNDA reactivity that ranged from strong to weak or negative. In advanced lesions, cells in the shoulder and those in fibrous tissue surrounding an atheroma were highly reactive for MNDA. However, only a fraction of the CD68 positive foam cells near the lipid core under the cap and shoulder contained MNDA reactivity. The variation in MNDA expression appeared to change with phenotypic specialization of monocytes in atherosclerosis consistent with its association with inflammation and suspected roles in regulating gene expression or in mediating cell death.

The death-domain fold of the ASC PYRIN domain, presenting a basis for PYRIN/PYRIN recognition

The PYRIN domain is a conserved sequence motif identified in more than 20 human proteins with putative functions in apoptotic and inflammatory signalling pathways. The three-dimensional structure of the PYRIN domain from human ASC was determined by NMR spectroscopy. The structure determination reveals close structural similarity to death domains, death effector domains, and caspase activation and recruitment domains, although the structural alignment with these other members of the death-domain superfamily differs from previously predicted amino acid sequence alignments. Two highly positively and negatively charged surfaces in the PYRIN domain of ASC result in a strong electrostatic dipole moment that is predicted to be present also in related PYRIN domains. These results suggest that electrostatic interactions play an important role for the binding between PYRIN domains. Consequently, the previously reported binding between the PYRIN domains of ASC and ASC2/POP1 or between the zebrafish PYRIN domains of zAsc and Caspy is proposed to involve interactions between helices 2 and 3 of one PYRIN domain with helices 1 and 4 of the other PYRIN domain, in analogy to previously reported homophilic interactions between caspase activation and recruitment domains.

Subcellular localization and mechanisms of nucleocytoplasmic distribution of p202, an interferon-inducible candidate for lupus susceptibility

Increased expression of p202 (52 kDa), an interferon (IFN)-inducible murine protein, in splenic cells (B- and T-cells) derived from female mice of the lupus-prone strains is correlated with increased susceptibility to develop systemic lupus erythematosus. However, the molecular mechanisms remain unclear. Our previous studies have indicated that, in IFN-treated fibroblasts, p202 is detected both in the cytoplasm and in the nucleus. Moreover, in the cytoplasm, a fraction of p202 associates with a membranous organelle. Here we report that, in the cytoplasm, a fraction of p202 associated with mitochondria. Additionally, we found that the constitutive p202 is primarily detected in the cytoplasm. Remarkably, the IFN treatment of cells potentiated nuclear accumulation of p202. Our observations are consistent with the possibility that IFN signaling regulates p202 levels as well as its nucleocytoplasmic distribution. These observations will serve as a basis to elucidate the molecular mechanisms by which p202 contributes to lupus susceptibility.

Homology modeling provides insights into the binding mode of the PAAD/DAPIN/pyrin domain, a fourth member of the CARD/DD/DED domain family

The PAAD/DAPIN/pyrin domain is the fourth member of the death domain superfamily, but unlike other members of this family, it is involved not only in apoptosis but also in innate immunity and several other processes. We have identified 40 PAAD domain-containing proteins by extensively searching the genomes of higher eukaryotes and viruses. Phylogenetic analyses suggest that there are five categories of PAAD domains that correlate with the domain architecture of the entire proteins. Homology models built on CARD and DD structures identified functionally important residues by studying conservation patterns on the surface of the models. Surface maps of each subfamily show different distributions of these residues, suggesting that domains from different subfamilies do not interact with each other, forming independent regulatory networks. Helix3 of PAAD is predicted to be critical for dimerization. Multiple alignment analysis and modeling suggest that it may be partly disordered, following a new paradigm for interaction proteins that are stabilized by protein-protein interactions.

The DAPIN family: a novel domain links apoptotic and interferon response proteins

We report the discovery of a protein domain, hereafter referred to as DAPIN, in diverse vertebrate and viral proteins that is associated with tumor biology, apoptosis and inflammation. Based on a secondary structure prediction, we suggest an all-alpha fold for DAPIN, which is also adopted by apoptotic protein domains of the CARD, death domain and death effector domain type.

PAAD - a new protein domain associated with apoptosis, cancer and autoimmune diseases

A new protein domain was found in several proteins involved in apoptosis, inflammation, cancer and immune responses. Its location within these proteins and predicted fold suggests that it functions as a protein-protein interaction domain, possibly uniting different signaling pathways.

p202 self-associates through a sequence conserved among the members of the 200-family proteins

Murine p202 is an interferon-inducible primarily nuclear phosphoprotein (52 kDa) whose expression in transfected cells inhibits colony formation. p202-binding proteins include the pocket proteins (pRb, p107 and p130), a p53-binding protein (sm53BP1), and transcription factors (e.g. NF-kappaB (p50 and p65), AP-1 (c-Fos and c-Jun), E2F-1, E2F-4, MyoD, and myogenin). p202 modulates the transcriptional activity of these factors in transfected cells. Here we demonstrate that p202 self-associates directly and a sequence in p202, which is conserved among the members of the 200-family proteins, was sufficient for self-association in vitro. Our observations reported herein raise the possibility that self-association of p202 may provide a mechanism for the regulation of its activity.

The Ifi 200 genes: an emerging family of IFN-inducible genes

The biological activities of interferons (IFNs) are mediated by IFN-induced proteins. One family is encoded by several structurally related genes located on murine chromosome 1 (Ifi 200 cluster) and three homologous genes (MNDA, IFI 16 and AIM2) located on human chromosome 1 as well, within a linkage group highly conserved between mouse and human. All the proteins of this family contain at least one copy of a conserved 200 amino acid domain, in addition to other regions that are different or missing among the various family members. Conservation of the 200 amino acid segment, therefore, may be responsible for a common function, while individually expressed domains may afford other tissue- or cell-specific functions. The data available demonstrate that at least two members of the Ifi 200 protein family, p202 and p204, inhibit cell proliferation in vitro. Moreover, high constitutive levels of p204 expression impair normal embryo development in transgenic animals. Here, we will review the principal features of murine and human proteins belonging to this family and their function in the cell growth-regulatory activities mediated by IFNs.