Molecular characterization of porcine SARM1 and its role in regulating TLRs signaling during highly pathogenic porcine reproductive and respiratory syndrome virus infection in vivo

The curious case of SARM1: Dr. Jekyll and Mr. Hyde in cell death and immunity?

Sterile alpha and toll/interleukin-1 receptor motif-containing protein 1 (SARM1) was first identified as a novel ortholog of Drosophila protein CG7915 and was subsequently placed as the fifth member of the human TIR-containing adaptor protein. SARM1 holds a unique position in this family where, unlike other members, it downregulates NFκB activity in response to immunogenic stimulation, interacts with another member of the family, TRIF, to negatively regulate its function, and it also mediates cell death responses. Over the past decade, SARM1 has emerged as one of the primary mediators of programmed axonal degeneration and this robust regulation of axonal degeneration-especially in models of peripheral neuropathy and traumatic injury-makes it an attractive target for therapeutic intervention. The TIR domain of SARM1 possesses an intrinsic NADase activity resulting in cellular energy deficits within the axons, a striking deviation from its other family members of human TLR adaptors. Interestingly, the TIR NADase activity, as seen in SARM1, is also observed in several prokaryotic TIR-containing proteins where they are involved in immune evasion once within the host. Although the immune function of SARM1 is yet to be conclusively discerned, this closeness in function with the prokaryotic TIR-domain containing proteins, places it at an interesting juncture of evolution raising questions about its origin and function in cell death and immunity. In this review, we discuss how a conserved immune adaptor protein like SARM1 switches to a pro-neurodegenerative function and the evolutionarily significance of the process.

A Novel NAD Signaling Mechanism in Axon Degeneration and its Relationship to Innate Immunity

Axon degeneration represents a pathological feature of many neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease where axons die before the neuronal soma, and axonopathies, such as Charcot-Marie-Tooth disease and hereditary spastic paraplegia. Over the last two decades, it has slowly emerged that a central signaling pathway forms the basis of this process in many circumstances. This is an axonal NAD-related signaling mechanism mainly regulated by the two key proteins with opposing roles: the NAD-synthesizing enzyme NMNAT2, and SARM1, a protein with NADase and related activities. The crosstalk between the axon survival factor NMNAT2 and pro-degenerative factor SARM1 has been extensively characterized and plays an essential role in maintaining the axon integrity. This pathway can be activated in necroptosis and in genetic, toxic or metabolic disorders, physical injury and neuroinflammation, all leading to axon pathology. SARM1 is also known to be involved in regulating innate immunity, potentially linking axon degeneration to the response to pathogens and intercellular signaling. Understanding this NAD-related signaling mechanism enhances our understanding of the process of axon degeneration and enables a path to the development of drugs for a wide range of neurodegenerative diseases.

SARM: From immune regulator to cell executioner

SARM is the fifth and most conserved member of the Toll/Il-1 Receptor (TIR) adaptor family. However, unlike the other TIR adaptors, MyD88, Mal, TRIF and TRAM, SARM does not participate in transducing signals downstream of TLRs. By contrast SARM inhibits TLR signalling by interacting with the adaptors TRIF and MyD88. In addition, SARM also has positive roles in innate immunity by activating specific transcriptional programs following immune challenge. SARM has a pivotal role in activating different forms of cell death following cellular stress and viral infection. Many of these functions of mammalian SARM are also reflected in SARM orthologues in lower organisms such as C. elegans and Drosophila. SARM expression is particularly enriched in neurons of the CNS and SARM has a critical role in neuronal death and in axon degeneration. Recent fascinating molecular insights have been revealed as to the molecular mechanism of SARM mediated axon degeneration. SARM has been shown to deplete NAD+ by possessing intrinsic NADase activity in the TIR domain of the protein. This activity can be activated experimentally by forced dimerization of the TIR domain. It is thought that this activity of SARM is normally switched off by the axo-protective activities of NMNAT2 which maintain low levels of the NAD+ precursor NMN. Therefore, there is now great excitement in the field of SARM research as targeting this enzymatic activity of SARM may lead to the development of new therapies for neurodegenerative diseases such as multiple sclerosis and motor neuron disease.

Beyond TLR Signaling—The Role of SARM in Antiviral Immune Defense, Apoptosis & Development

SARM (Sterile alpha and armadillo motif-containing protein) is the recently identified TIR domain-containing cytosolic protein. Classified as a member of the TLR adaptor family, the multiple locations and functions of SARM (sometimes playing opposing roles), provoke an enigma on its biology. Although originally assumed to be a member of the TLR adaptor family (functioning as a negative regulator of TLR signaling pathway), latest findings indicate that SARM regulates signaling differently from other TLR adaptor proteins. Recent studies have highlighted the significant functional role of SARM in mediating apoptosis and antiviral innate immune response. In this review, we provide an update on the evolutionary conservation, spatial distribution, and regulated expression of SARM to highlight its diverse functional roles. The review will summarize findings on the known interacting partners of SARM and provide analogy on how they add new dimensions to the current understanding on the multifaceted roles of SARM in antiviral activities and apoptotic functions. In addition, we provide a future perspective on the roles of SARM in differentiation and development, with substantial emphasis on the molecular insights to its mechanisms of action.

Expression of Toll-like receptor signaling-related genes in pigs co-infected with porcine reproductive and respiratory syndrome virus and porcine circovirus type 2

Pigs co-infected with porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2) have been shown to develop more severe diseases than pigs infected with PRRSV or PCV2 only. The underlying interaction mechanisms between the two viruses in developing the disease are unclear. The present study investigates the mRNA expression of Toll-like receptor (TLR) signaling-related molecules in peripheral blood mononuclear cells from pigs infected with PRRSV or PCV2 or both. The mRNA expression levels were determined by quantitative real-time RT-PCR. Co-infection of pigs with PRRSV and PCV2 resulted in a negatively synergistic effect on the mRNA expression of the negative regulators of TLR, including A20, Bcl-3, IRAK-M, MKP-1, SARM1 and SIGIRR, as well as the TLR downstream transcription factors IRF-1 and IRF-3. A positively synergistic effect of a combined infection of PRRSV and PCV2 on the CD14 mRNA expression was also observed.

Hm-MyD88 and Hm-SARM: two key regulators of the neuroimmune system and neural repair in the medicinal leech

Unlike mammals, the CNS of the medicinal leech can regenerate damaged neurites, thus restoring neural functions after lesion. We previously demonstrated that the injured leech nerve cord is able to mount an immune response promoting the regenerative processes. Indeed neurons and microglia express sensing receptors like Hm-TLR1, a leech TLR ortholog, associated with chemokine release in response to a septic challenge or lesion. To gain insights into the TLR signaling pathways involved during these neuroimmune responses, members of the MyD88 family were investigated. In the present study, we report the characterization of Hm-MyD88 and Hm-SARM. The expression of their encoding gene was strongly regulated in leech CNS not only upon immune challenge but also during CNS repair, suggesting their involvement in both processes. This work also showed for the first time that differentiated neurons of the CNS could respond to LPS through a MyD88-dependent signalling pathway, while in mammals, studies describing the direct effect of LPS on neurons and the outcomes of such treatment are scarce and controversial. In the present study, we established that this PAMP induced the relocalization of Hm-MyD88 in isolated neurons.

Grass carp SARM1 and its two splice variants negatively regulate IFN-I response and promote cell death upon GCRV infection at different subcellular locations

Sterile alpha and Toll/IL-1R motif containing 1 (SARM1) negatively regulates TRIF-dependent TLR signaling in mammals. However, its immune function remains unclear in teleost. Here, a grass carp Ctenopharyngodon idella SARM1 (CiSARM1) gene and its two novel splice variants (CiSARM1s1 and CiSARM1s2) were identified. CiSARM1s1 and CiSARM1s2 are generated by intron retention mechanism, and they only retain N-terminal HEAT/armadillo motifs. In C. idella kidney (CIK) cells, CiSARM1 and CiSARM1s1 are located in mitochondria, whereas CiSARM1s2 distributes in the whole cell. All the three transcripts are ubiquitously expressed in 15 investigated tissues. They were responsive to GCRV in vivo and in vitro and to viral/bacterial PAMPs in vitro, implying they participate in both antiviral and antibacterial immune responses. By overexpression experiment, CiSARM1 and its two isoforms affected each other's expression in CIK cells. CiSARM1 inhibited GCRV-triggered IFN-I response by affecting the expressions of CiTRIF, CiMyD88, CiIPS-1, CiTRAF6, CiTBK1, CiIRF3 and CiIRF7 in TRIF-, MyD88- and IPS-1-dependent pathways; CiSARM1s1 and CiSARM1s2 inhibited GCRV-triggered IFN-I production through suppressing the expressions of CiMyD88, CiIPS-1, CiTRAF6, CiTBK1, CiIRF3 and CiIRF7 in MyD88- and IPS-1-dependent pathways. Moreover, antiviral activity assays indicated that all the three genes promote GCRV-induced cell death. These results were further verified by RNAi experiments. Thus, CiSARM1 and its two splice variants jointly prevent excessive activation of the host immune response. These findings uncover the regulatory mechanisms of SARM1 in teleost and lay a foundation for further functional and evolutionary researches on SARM1.

Molecular characterization of the porcine S100A6 gene and analysis of its expression in pigs infected with highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV)

Our previous microarray study revealed that S100A6 was significantly upregulated in porcine alveolar macrophages (PAMs) infected with highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV). In the present study, we cloned both cDNA and genomic DNA sequences of the gene. Transient transfection indicated that the porcine S100A6 protein was located in the nucleus and cytoplasm. Reverse transcription polymerase chain reaction (RT-PCR) revealed that the porcine S100A6 gene was highly expressed in the kidney and subcutaneous fat. Polyinosinic-polycytidylic acid [poly (I:C)] induced porcine S100A6 gene expression in PK-15 cells. Quantitative real-time PCR (Q-PCR) analysis further showed that the porcine S100A6 gene was upregulated in different cells and tissues of Tongcheng pigs infected with HP-PRRSV. Chromosome walking obtained the porcine S100A6 promoter region and then luciferase reporter assays confirmed its regulatory activities. We observed a putative NF-κB binding site in the core promoter region, which may explain the upregulation of porcine S100A6 in response to PRRSV. Transfection of NF-κB (p65 subunit) intensely induced the promoter activity of the porcine S100A6 gene, while an NF-κB inhibitor, pyrrolidine dithiocarbamate (PDTC), inhibited this activity. Furthermore, compared to its wild type, the promoter activity was significantly reduced when it contained a mutant NF-κB binding site. All these results provide a solid foundation to further investigate how S100A6 is involved in PRRSV infection.

The porcine innate immune system: an update

Over the last few years, we have seen an increasing interest and demand for pigs in biomedical research. Domestic pigs (Sus scrofa domesticus) are closely related to humans in terms of their anatomy, genetics, and physiology, and often are the model of choice for the assessment of novel vaccines and therapeutics in a preclinical stage. However, the pig as a model has much more to offer, and can serve as a model for many biomedical applications including aging research, medical imaging, and pharmaceutical studies to name a few. In this review, we will provide an overview of the innate immune system in pigs, describe its anatomical and physiological key features, and discuss the key players involved. In particular, we compare the porcine innate immune system to that of humans, and emphasize on the importance of the pig as model for human disease.

Interferon induced IFIT family genes in host antiviral defense

Secretion of interferons (IFNs) from virus-infected cells is a hallmark of host antiviral immunity and in fact, IFNs exert their antiviral activities through the induction of antiviral proteins. The IFN-induced protein with tetratricopeptide repeats (IFITs) family is among hundreds of IFN-stimulated genes. This family contains a cluster of duplicated loci. Most mammals have IFIT1, IFIT2, IFIT3 and IFIT5; however, bird, marsupial, frog and fish have only IFIT5. Regardless of species, IFIT5 is always adjacent to SLC16A12. IFIT family genes are predominantly induced by type I and type III interferons and are regulated by the pattern recognition and the JAK-STAT signaling pathway. IFIT family proteins are involved in many processes in response to viral infection. However, some viruses can escape the antiviral functions of the IFIT family by suppressing IFIT family genes expression or methylation of 5' cap of viral molecules. In addition, the variants of IFIT family genes could significantly influence the outcome of hepatitis C virus (HCV) therapy. We believe that our current review provides a comprehensive picture for the community to understand the structure and function of IFIT family genes in response to pathogens in human, as well as in animals.

T-cell death following immune activation is mediated by mitochondria-localized SARM

Following acute-phase infection, activated T cells are terminated to achieve immune homeostasis, failure of which results in lymphoproliferative and autoimmune diseases. We report that sterile α- and heat armadillo-motif-containing protein (SARM), the most conserved Toll-like receptors adaptor, is proapoptotic during T-cell immune response. SARM expression is significantly reduced in natural killer (NK)/T lymphoma patients compared with healthy individuals, suggesting that decreased SARM supports NK/T-cell proliferation. T cells knocked down of SARM survived and proliferated more significantly compared with wild-type T cells following influenza infection in vivo. During activation of cytotoxic T cells, the SARM level fell before rising, correlating inversely with cell proliferation and subsequent T-cell clearance. SARM knockdown rescued T cells from both activation- and neglect-induced cell deaths. The mitochondria-localized SARM triggers intrinsic apoptosis by generating reactive oxygen species and depolarizing the mitochondrial potential. The proapoptotic function is attributable to the C-terminal sterile alpha motif and Toll/interleukin-1 receptor domains. Mechanistically, SARM mediates intrinsic apoptosis via B cell lymphoma-2 (Bcl-2) family members. SARM suppresses B cell lymphoma-extra large (Bcl-xL) and downregulates extracellular signal-regulated kinase phosphorylation, which are cell survival effectors. Overexpression of Bcl-xL and double knockout of Bcl-2 associated X protein and Bcl-2 homologous antagonist killer substantially reduced SARM-induced apoptosis. Collectively, we have shown how T-cell death following infection is mediated by SARM-induced intrinsic apoptosis, which is crucial for T-cell homeostasis.

Experimental infection of United States swine with a Chinese highly pathogenic strain of porcine reproductive and respiratory syndrome virus

The pathogenesis of Type 2 highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) in 10-week old swine in the United States was investigated. rJXwn06, rescued from an infectious clone of Chinese HP-PRRSV, replicated in swine with at least 100-fold increased kinetics over U.S. strain VR-2332. rJXwn06 caused significant weight loss, exacerbated disease due to bacterial sepsis and more severe histopathological lung lesions in pigs exposed to HP-PRRSV than to those infected with VR-2332. Novel findings include identification of bacterial species present, the degree of thymic atrophy seen, and the inclusion of contact animals that highlighted the ability of HP-PRRSV to rapidly transmit between animals. Furthermore, comprehensive detailed cytokine analysis of serum, bronchoalveolar lavage fluid, and tracheobronchial lymph node tissue homogenate revealed a striking elevation in levels of cytokines associated with both innate and adaptive immunity in HP-PRRSV infected swine, and showed that contact swine differed in the degree of cytokine response.