Nicolas Zeller

A Lymphotoxin-Driven Pathway to Hepatocellular Carcinoma

Hepatitis B and C viruses (HBV and HCV) cause chronic hepatitis and hepatocellular carcinoma (HCC) by poorly understood mechanisms. We show that cytokines lymphotoxin (LT) alpha and beta and their receptor (LTbetaR) are upregulated in HBV- or HCV-induced hepatitis and HCC. Liver-specific LTalphabeta expression in mice induces liver inflammation and HCC, causally linking hepatic LT overexpression to hepatitis and HCC. Development of HCC, composed in part of A6(+) oval cells, depends on lymphocytes and IKappa B kinase beta expressed by hepatocytes but is independent of TNFR1. In vivo LTbetaR stimulation implicates hepatocytes as the major LT-responsive liver cells, and LTbetaR inhibition in LTalphabeta-transgenic mice with hepatitis suppresses HCC formation. Thus, sustained LT signaling represents a pathway involved in hepatitis-induced HCC.

TAK1 Suppresses a NEMO-Dependent but NF-κB-Independent Pathway to Liver Cancer

The MAP3-kinase TGF-beta-activated kinase 1 (TAK1) critically modulates innate and adaptive immune responses and connects cytokine stimulation with activation of inflammatory signaling pathways. Here, we report that conditional ablation of TAK1 in liver parenchymal cells (hepatocytes and cholangiocytes) causes hepatocyte dysplasia and early-onset hepatocarcinogenesis, coinciding with biliary ductopenia and cholestasis. TAK1-mediated cancer suppression is exerted through activating NF-kappaB in response to tumor necrosis factor (TNF) and through preventing Caspase-3-dependent hepatocyte and cholangiocyte apoptosis. Moreover, TAK1 suppresses a procarcinogenic and pronecrotic pathway, which depends on NF-kappaB-independent functions of the I kappaB-kinase (IKK)-subunit NF-kappaB essential modulator (NEMO). Therefore, TAK1 serves as a gatekeeper for a protumorigenic, NF-kappaB-independent function of NEMO in parenchymal liver cells.

Early and rapid engraftment of bone marrow-derived microglia in scrapie.

Prion neuroinvasion is accompanied by maximal activation of microglia, the significance of which for pathogenesis is unknown. Here, we used bone marrow (BM) cells expressing GFP (green fluorescent protein) to study the turnover of microglia in mouse scrapie. We found that ≥50% of all brain microglia were replaced by BM-derived cells before clinical disease onset. In terminally sick mice, microglia density increased threefold to fourfold. Hence BM-derived microglia rapidly and efficaciously colonize the brain in scrapie. Whereas reconstitution of wild-type mice with prion protein-deficient (Prnpo/o) BM did not alter scrapie pathogenesis, Prnpo/o mice transplanted with wild-type BM cells were resistant to peripherally administered prions despite high levels of infectivity in the spleen. Cerebellar homogenates from prion-inoculated Prnpo/o mice reconstituted with >10% of wild-type microglia failed to infect transgenic mice overexpressing the cellular prion protein. Hence, in contrast to previous reports, microglia are not competent for efficient prion transport and replication in vivo.

Stromal Complement Receptor CD21/35 Facilitates Lymphoid Prion Colonization and Pathogenesis

We have studied the role of CD21/35, which bind derivatives of complement factors C3 and C4, in extraneural prion replication and neuroinvasion. Upon administration of small prion inocula, CD21/35−/− mice experienced lower attack rates and delayed disease over both wild-type (WT) mice and mice with combined C3 and C4 deficiencies. Early after inoculation, CD21/35−/− spleens were devoid of infectivity. Reciprocal adoptive bone marrow transfers between WT and CD21/35−/− mice revealed that protection from prion infection resulted from ablation of stromal, but not hemopoietic, CD21/35. Further adoptive transfer experiments between WT mice and mice devoid of both the cellular prion protein PrPC and CD21/35 showed that splenic retention of inoculum depended on stromal CD21/35 expression. Because both PrPC and CD21/35 are highly expressed on follicular dendritic cells, CD21/35 appears to be involved in targeting prions to follicular dendritic cells and expediting neuroinvasion following peripheral exposure to prions.

IκB kinase 2 determines oligodendrocyte loss by non-cell-autonomous activation of NF-κB in the central nervous system

The IκB kinase complex induces nuclear factor kappa B activation and has recently been recognized as a key player of autoimmunity in the central nervous system. Notably, IκB kinase/nuclear factor kappa B signalling regulates peripheral myelin formation by Schwann cells, however, its role in myelin formation in the central nervous system during health and disease is largely unknown. Surprisingly, we found that brain-specific IκB kinase 2 expression is dispensable for proper myelin assembly and repair in the central nervous system, but instead plays a fundamental role for the loss of myelin in the cuprizone model. During toxic demyelination, inhibition of nuclear factor kappa B activation by conditional ablation of IκB kinase 2 resulted in strong preservation of central nervous system myelin, reduced expression of proinflammatory mediators and a significantly attenuated glial response. Importantly, IκB kinase 2 depletion in astrocytes, but not in oligodendrocytes, was sufficient to protect mice from myelin loss. Our results reveal a crucial role of glial cell-specific IκB kinase 2/nuclear factor kappa B signalling for oligodendrocyte damage during toxic demyelination. Thus, therapies targeting IκB kinase 2 function in non-neuronal cells may represent a promising strategy for the treatment of distinct demyelinating central nervous system diseases.

USP18 lack in microglia causes destructive interferonopathy of the mouse brain

Microglia are tissue macrophages of the central nervous system (CNS) that control tissue homeostasis. Microglia dysregulation is thought to be causal for a group of neuropsychiatric, neurodegenerative and neuroinflammatory diseases, called “microgliopathies”. However, how the intracellular stimulation machinery in microglia is controlled is poorly understood. Here, we identified the ubiquitin‐specific protease (Usp) 18 in white matter microglia that essentially contributes to microglial quiescence. We further found that microglial Usp18 negatively regulates the activation of Stat1 and concomitant induction of interferon‐induced genes, thereby terminating IFN signaling. The Usp18‐mediated control was independent from its catalytic activity but instead required the interaction with Ifnar2. Additionally, the absence of Ifnar1 restored microglial activation, indicating a tonic IFN signal which needs to be negatively controlled by Usp18 under non‐diseased conditions. These results identify Usp18 as a critical negative regulator of microglia activation and demonstrate a protective role of Usp18 for microglia function by regulating the Ifnar pathway. The findings establish Usp18 as a new molecule preventing destructive microgliopathy.

The unexpected role of lymphotoxin |[beta]| receptor signaling in carcinogenesis: from lymphoid tissue formation to liver and prostate cancer development

The cytokines lymphotoxin (LT) α, β and their receptor (LTβR) belong to the tumor necrosis factor (TNF) superfamily, whose founder—TNFα—was initially discovered due to its tumor necrotizing activity. LTβR signaling serves pleiotropic functions including the control of lymphoid organ development, support of efficient immune responses against pathogens due to maintenance of intact lymphoid structures, induction of tertiary lymphoid organs, liver regeneration or control of lipid homeostasis. Signaling through LTβR comprises the noncanonical/canonical nuclear factor-κB (NF-κB) pathways thus inducing chemokine, cytokine or adhesion molecule expression, cell proliferation and cell survival. Blocking LTβR signaling or Fcγ-receptor mediated immunoablation of LT-expressing cells was demonstrated to be beneficial in various infectious or noninfectious inflammatory or autoimmune disorders. Only recently, LTβR signaling was shown to initiate inflammation-induced carcinogenesis, to influence primary tumorigenesis and to control reemergence of carcinoma in various cancer models through distinct mechanisms. Indeed, LTβR signaling inhibition has already been used as efficient anti-inflammatory, anti-cancer therapy in some experimental models. Here, we review the pleiotropic functions attributed to LT, the effects of its deregulation and extensively discuss the recent literature on LTs link to carcinogenesis.

Lymphotoxin-Dependent Prion Replication in Inflammatory Stromal Cells of Granulomas

Prior to invading the nervous system, prions frequently colonize lymphoid organs and sites of inflammatory lymphoneogenesis, where they colocalize with Mfge8+ follicular dendritic cells (FDCs). Here, we report that soft-tissue granulomas, a frequent feature of chronic inflammation, expressed the cellular prion protein (PrPC, encoded by Prnp) and the lymphotoxin receptor (LTbetaR), even though they lacked FDCs and did not display lymphoneogenesis. After intraperitoneal prion inoculation, granulomas of Prnp(+/+) mice, but not Prnp(-/-) granulomas or unaffected Prnp(+/+) skin, accumulated prion infectivity and disease-associated prion protein. Bone-marrow transfers between Prnp(+/+) and Prnp(-/-) mice and administration of lymphotoxin signaling antagonists indicated that prion replication required radioresistant PrPC-expressing cells and LTbetaR signaling. Granulomatous PrPC was mainly expressed by stromal LTbetaR+ mesenchymal cells that were absent from unaffected subcutis. Hence, granulomas can act as clinically silent reservoirs of prion infectivity. Furthermore, lymphotoxin-dependent prion replication can occur in inflammatory stromal cells that are distinct from FDCs.

Expression of lymphotoxin beta governs immunity at two distinct levels.

Interaction of lymphotoxin α1β2 (LTα1β2) with its receptor is key for the generation and maintenance of secondary lymphoid organ microstructure. We used mice conditionally deficient for LTβ on different lymphocyte subsets to determine how the LTβ‐dependent lymphoid structure influences immune reactivity. All conditionally LTβ‐deficient mice mounted normal immune responses against vesicular stomatitis virus (VSV), and were protected against lymphocytic choriomeningitis virus (LCMV). In contrast, they exhibited reduced immune responses against non‐replicating antigens. Completely LTβ‐deficient mice failed to retain VSV in the marginal zone and died from VSV infections, and they became virus carriers following infection with the non‐cytopathic LCMV, which was correlated with defective virus replication in dendritic cells. It was ruled out that LTβ expression on lymphocytes influenced their activation, homing capacity, or maturation. We therefore conclude that LTβ expression influences immune reactivity at two distinct levels: (i) Expression of LTβ on lymphocytes enhances the induction of immune responses against limiting amounts of antigen. (ii) Expression of LTβ on non‐lymphocytes governs antiviral immunity by enhancing antigen presentation on antigen‐presenting cells. This prevents cytotoxic T lymphocytes exhaustion or death of the host by uncontrolled virus spread.

The parallel universe: microRNAs and their role in chronic hepatitis, liver tissue damage and hepatocarcinogenesis

In recent years, enormous progress has been made in identifying microRNAs (miRNAs) as important regulators of gene expression and their association with or control of various liver diseases such as fibrosis, hepatitis and hepatocellular carcinoma (HCC). Indeed, many genes encoding miRNAs as well as their targets have been described and their direct or indirect link to the respective liver diseases has been investigated in various experimental systems as well as in human tissue. Here we discuss current knowledge of miRNAs and their involvement in liver diseases, elaborating in particular on the contribution of miRNAs to hepatitis, fibrosis and HCC formation. We also debate possible prognostic, predictive and therapeutic values of respective miRNAs in liver diseases. The discovery of liver disease related miRNAs has constituted a major breakthrough in liver research and will most likely be of high relevance for future therapeutic strategies, especially when dealing with hepatitis, fibrosis and HCC.