Receptor-interacting protein (RIP) kinase family

LRRK2 levels in immune cells are increased in Parkinson's disease

Mutations associated with leucine-rich repeat kinase 2 are the most common known cause of Parkinson’s disease. The known expression of leucine-rich repeat kinase 2 in immune cells and its negative regulatory function of nuclear factor of activated T cells implicates leucine-rich repeat kinase 2 in the development of the inflammatory environment characteristic of Parkinson’s disease. The aim of this study was to determine the expression pattern of leucine-rich repeat kinase 2 in immune cell subsets and correlate it with the immunophenotype of cells from Parkinson’s disease and healthy subjects. For immunophenotyping, blood cells from 40 Parkinson’s disease patients and 32 age and environment matched-healthy control subjects were analyzed by flow cytometry. Multiplexed immunoassays were used to measure cytokine output of stimulated cells. Leucine-rich repeat kinase 2 expression was increased in B cells (p = 0.0095), T cells (p = 0.029), and CD16⁺ monocytes (p = 0.01) of Parkinson’s disease patients compared to healthy controls. Leucine-rich repeat kinase 2 induction was also increased in monocytes and dividing T cells in Parkinson’s disease patients compared to healthy controls. In addition, Parkinson’s disease patient monocytes secreted more inflammatory cytokines compared to healthy control, and cytokine expression positively correlated with leucine-rich repeat kinase 2 expression in T cells from Parkinson’s disease but not healthy controls. Finally, the regulatory surface protein that limits T-cell activation signals, CTLA-4 (cytotoxic T-lymphocyte-associated protein 4), was decreased in Parkinson’s disease compared to HC in T cells (p = 0.029). In sum, these findings suggest that leucine-rich repeat kinase 2 has a regulatory role in immune cells and Parkinson’s disease. Functionally, the positive correlations between leucine-rich repeat kinase 2 expression levels in T-cell subsets, cytokine expression and secretion, and T-cell activation states suggest that targeting leucine-rich repeat kinase 2 with therapeutic interventions could have direct effects on immune cell function.

High levels of leucine-rich repeat kinase 2 (LRRK2) in immune cells disrupt immune system function in patients with Parkinson’s disease (PD). Mutations in LRRK2 are the most common genetic cause of PD. Although LRRK2 is found in many immune cells, research efforts have focussed on determining its effects on neuronal function. Malu G. Tansey at Emory University, USA, and colleagues compared the levels and function of LRKK2 in immune cells from 40 late-onset PD patients and 32 age- and environment-matched healthy controls. The cells from PD patients had higher levels of LRKK2 protein and produced more pro-inflammatory molecules in response to stimulation than the control cells. As exacerbated inflammatory responses are known to aggravate neurodegeneration, monitoring LRKK2 levels may aid the assessment of disease progression in both inherited and sporadic cases of PD.

The Receptor-interacting Serine/Threonine Protein Kinase 1 (RIPK1) Regulates Progranulin Levels

Progranulin (PGRN), a secreted growth factor, is a key regulator of inflammation and is genetically linked to two common and devastating neurodegenerative diseases. Haploinsufficiency mutations in GRN, the gene encoding PGRN, cause frontotemporal dementia (FTD), and a GRN SNP confers significantly increased risk for Alzheimer's disease (AD). Because cellular and animal data indicate that increasing PGRN can reverse phenotypes of both FTD and AD, modulating PGRN level has been proposed as a therapeutic strategy for both diseases. However, little is known about the regulation of PGRN levels. In this study, we performed an siRNA-based screen of the kinome to identify genetic regulators of PGRN levels in a rodent cell-based model system. We found that knocking down receptor-interacting serine/threonine protein kinase 1 (Ripk1) increased both intracellular and extracellular PGRN protein levels by increasing the translation rate of PGRN without affecting mRNA levels. We observed this effect in Neuro2a cells, wild-type primary mouse neurons, and Grn-haploinsufficient primary neurons from an FTD mouse model. We found that the effect of RIPK1 on PGRN is independent of the kinase activity of RIPK1 and occurs through a novel signaling pathway. These data suggest that targeting RIPK1 may be a therapeutic strategy in both AD and FTD.

LRRK2 and the Immune System

Polymorphisms in leucine-rich repeat kinase 2 (LRRK2) have been linked to familial Parkinson's disease, increased risk of sporadic Parkinson's disease, increased risk of Crohn's inflammatory bowel disease, and increased susceptibility to leprosy. As well as LRRK2 mutations, these diseases share in common immune dysfunction and inflammation. LRRK2 is highly expressed in particular immune cells and has been biochemically linked to the intertwined pathways regulating inflammation, mitochondrial function, and autophagy/lysosomal function. This review outlines what is currently understood about LRRK2 function in the immune system and the potential implications of LRRK2 dysfunction for diseases genetically linked to this enigmatic enzyme.

Potential therapeutic targets for inflammation in toll-like receptor 4 (TLR4)-mediated signaling pathways

Inflammation is set off when innate immune cells detect infection or tissue injury. Tight control of the severity, duration, and location of inflammation is an absolute requirement for an appropriate balance between clearance of injured tissue and pathogens versus damage to host cells. Impeding the risk associated with the imbalance in the inflammatory response requires precise identification of potential therapeutic targets involved in provoking the inflammation. Toll-like receptors (TLRs) primarily known for the pathogen recognition and subsequent immune responses are being investigated for their pathogenic role in various chronic diseases. A mammalian homologue of Drosophila Toll receptor 4 (TLR4) was shown to induce the expression of genes involved in inflammatory responses. Signaling pathways via TLR4 activate various transcription factors like Nuclear factor kappa-light-chain-enhancer (NF-κB), activator protein 1 (AP1), Signal Transducers and Activators of Transcription family of transcription factors (STAT1) and Interferon regulatory factors (IRF's), which are the key players regulating the inflammatory response. Inhibition of these targets and their upstream signaling molecules provides a potential therapeutic approach to treat inflammatory diseases. Here we review the therapeutic targets involved in TLR-4 signaling pathways that are critical for suppressing chronic inflammatory disorders.

The role of NOD1 and NOD2 in host defense against chlamydial infection

Chlamydial species are common intracellular parasites that cause various diseases, mainly characterized by persistent infection, which lead to inflammatory responses modulated by pattern recognition receptors (PRRs). The best understood PRRs are the extracellular Toll-like receptors, but recent significant advances have focused on two important proteins, NOD1 and NOD2, which are members of the intracellular nucleotide-binding oligomerization domain receptor family and are capable of triggering the host innate immune signaling pathways. This results in the production of pro-inflammatory cytokines, which is vital for an adequate host defense against intracellular chlamydial infection. NOD1/2 ligands are known to derive from peptidoglycan, and the latest research has resolved the paradox of whether chlamydial species possess this bacterial cell wall component; this finding is likely to promote in-depth investigations into the interaction between the NOD proteins and chlamydial pathogens. In this review, we summarize the basic characteristics and signal transduction functions of NOD1 and NOD2 and highlight the new research on the roles of NOD1 and NOD2 in the host defense against chlamydial infection.

Dual Inhibition of Rip2 and IRAK1/4 Regulates IL-1β and IL-6 in Sarcoidosis Alveolar Macrophages and Peripheral Blood Mononuclear Cells

Sarcoidosis is a multisystem granulomatous disease of unknown etiology that primarily affects the lungs. Our previous work indicates that activation of p38 plays a pivotal role in sarcoidosis inflammatory response. Therefore, we investigated the upstream kinase responsible for activation of p38 in sarcoidosis alveolar macrophages (AMs) and PBMCs. We identified that sustained p38 phosphorylation in sarcoidosis AMs and PBMCs is associated with active MAPK kinase 4 but not with MAPK kinase 3/6. Additionally, we found that sarcoidosis AMs exhibit a higher expression of IRAK1, IRAK-M, and receptor interacting protein 2 (Rip2). Surprisingly, ex vivo treatment of sarcoidosis AMs or PBMCs with IRAK1/4 inhibitor led to a significant increase in IL-1β mRNA expression both spontaneously and in response to TLR2 ligand. However, a combination of Rip2 and IRAK-1/4 inhibitors significantly decreased both IL-1β and IL-6 production in sarcoidosis PBMCs and moderately in AMs. Importantly, a combination of Rip2 and IRAK-1/4 inhibitors led to decreased IFN-γ and IL-6 and decreased percentage of activated CD4(+)CD25(+) cells in PBMCs. These data suggest that in sarcoidosis, both pathways, namely IRAK and Rip2, are deregulated. Targeted modulation of Rip2 and IRAK pathways may prove to be a novel treatment for sarcoidosis.

RIP kinase-mediated ROS production triggers XAF1 expression through activation of TAp73 in casticin-treated bladder cancer cells

The p53 family protein p73 plays an important role in apoptosis induced by chemotherapeutic drugs. Transcriptionally active (TA) p73 (TAp73) substitutes for p53 in the response to stress. XIAP associated factor 1 (XAF1) is a novel predictive and prognostic factor in patients with bladder cancer, but the association between TAp73 and XAF1 expression in bladder cancer cells is poorly understood. Here, we investigated the status of TAp73 and XAF1 in T24 bladder cancer cells to identify molecular mechanisms in casticin‑exposed T24 cells. Casticin induced activation of JNK/p38 MAPK that preceded activation of the caspase cascade and disruption of the mitochondria membrane potential (∆ψm). Expression of XAF1 and TAp73 was also upregulated in casticin-treated T24 cells. Casticin treatment of T24 cells induced receptor-interacting protein (RIP) kinase expression and increased intracellular production of reactive oxygen species (ROS). Casticin-mediated ROS induced an increase in phosphorylated JNK/p38 MAPK, resulting in progressive upregulation of TAp73, which in turn led to XAF1 expression. Our data suggest that the apoptotic activity of casticin in T24 cells is mediated by activation of the TAp73-XAF1 signaling pathway through RIP kinase-mediated ROS production.

A critical role for the protein kinase PKK in the maintenance of recirculating mature B cells and the development of B1 cells

Protein kinase C associated kinase (PKK) regulates NF-κB activation and is required for the survival of certain lymphoma cells. Mice lacking PKK die soon after birth, and previous studies suggest that the role of PKK in B cell development might be context dependent. We have generated a mouse strain harboring conditional null alleles for PKK and a Cre-recombinase transgene under the control of the endogenous CD19 promoter. In the present study, we show that knockout of PKK in B cells results in the reduction of long-lived recirculating mature B cell population in lymph nodes and bone marrow as well as a decrease in peritoneal B1 cells, while PKK deficiency has no apparent effect on early B cell development in bone marrow or the development of follicular and marginal zone B cells in the spleen. In addition, we demonstrate that PKK-deficient B cells display defective proliferation and survival responses to stimulation of B cell receptor (BCR), which may underlie the reduction of recirculating mature B cells in PKK mutant mice. Consistently, BCR-mediated NF-κB activation, known to be required for the survival of activated but not resting B cells, is attenuated in PKK-deficient B cells. Thus, our results reveal a critical role of PKK in the maintenance of recirculating mature B cells as well as the development of B1 cells in mice.

Function and dysfunction of leucine-rich repeat kinase 2 (LRRK2): Parkinson's disease and beyond

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of familial Parkinson’s disease (PD). As such, functions and dysfunctions of LRRK2 in PD have been the subject of extensive investigation. In addition to PD, increasing evidence is suggesting that LRRK2 is associated with a wide range of diseases. Genome-wide association studies have implicated LRRK2 in Crohn’s disease (CD) and leprosy, and the carriers with pathogenic mutations of LRRK2 show increased risk to develop particular types of cancer. LRRK2 mutations are rarely found in Alzheimer’s disease (AD), but LRRK2 might play a part in tauopathies. The association of LRRK2 with the pathogenesis of apparently unrelated diseases remains enigmatic, but it might be related to the yet unknown diverse functions of LRRK2. Here, we reviewed current knowledge on the link between LRRK2 and several diseases, including PD, AD, CD, leprosy, and cancer, and discussed the possibility of targeting LRRK2 in such diseases. [BMB Reports 2015; 48(5): 243-248]

Targeting Cell Death and Sterile Inflammation Loop for the Treatment of Nonalcoholic Steatohepatitis

Nonalcoholic fatty liver disease represents a wide spectrum of conditions and is currently the most common form of chronic liver disease affecting both adults and children in the United States and many other parts of the world. Great effort has been focused on the development of novel therapies for those patients with the more advanced forms of the disease, in particular those with nonalcoholic steatohepatitis (NASH) and liver fibrosis that can be associated with significant morbidity and mortality. In this review, the authors focus on the role of cell death and sterile inflammatory pathways as well as the self-perpetuating deleterious cycle they may trigger as novel therapeutic targets for the treatment of fibrotic NASH.

Evaluation of RIP1K and RIP3K expressions in the malignant and benign breast tumors

Receptor-interacting protein kinase 1 (RIP1K) and RIP3K belong to RIPK family, which regulate cell survival and cell death. In the present investigation, the expression levels of RIP1K and RIP3K were evaluated in the 30 malignant, 15 benign, and 20 normal breast tissues, and their correlation with clinicopathological characteristics was also studied. The expression levels of RIP1K and RIP3K were determined, by western blot analysis. The relative RIP1K expression was significantly higher in the malignant and benign tumors when compared to those of normal tissues (P < 0.0001 and P < 0.001, respectively). However, the expression level of RIP3K was significantly lower in the malignant tumors than those of normal and benign values (P < 0.001 and P < 0.01, respectively). Positive significant correlation was found for RIP1K expression with tumor size (P < 0.001), grades (P < 0.0001), and c-erbB2 (P < 0.001), but negative significant correlation was detected with patient's age (P < 0.001), estrogen receptor (ER) (P < 0.001), progesterone receptor (PR) (P < 0.01), and P53 (P<0.01) status. RIP3K expression was significantly lower in the pre-menopauses (P < 0.01), grade III (P < 0.05), ER-negative (P < 0.05), and c-erbB2-negative malignant tumors, but no correlation was detected with tumor size, PR, and P53 status. No significant correlation was observed for RIP1K and RIP3K expressions with Ki67 and Her2. Based on the present results, it is concluded that reduction of RIP3K expression in the malignant breast tumor might be an important evidence to support the antitumor activity of this enzyme in vivo. However, RIP1K expression was shown to be higher in the malignant breast tumors than those of normal and benign breast tissues, which probably designates as a poor prognostic factor.

PAX5 interacts with RIP2 to promote NF-κB activation and drug-resistance of B-lymphoproliferative disorders

Paired box protein 5 (PAX5) plays a lineage determination role in B-cell development. However, high expression of PAX5 has been also found in various malignant diseases including B-lymphoproliferative disorders (B-LPDs), but its functions and mechanisms in these diseases are still unclear. Here, we show that PAX5 induces drug-resistance through association and activation of receptor-interacting serine/threonine-protein kinase2 (RIP2) and subsequent activation of NF-κB signaling and anti-apoptosis genes expression in B-lymphoproliferative cells. Furthermore, PAX5 is able to interact with RIP1-3, modulating both RIP1- mediated TNFR and RIP2-mediated NOD1 and NOD2 pathways. Our findings describe a novel function of PAX5 in regulating RIP1 and RIP2 activation, which is at least involved in chemo drug-resistance in B-LPDs.

Unusual Suspects in the Twilight Zone Between the Hsp90 Interactome and Carcinogenesis

The molecular chaperone Hsp90 has attracted a lot of interest in cancer research ever since cancer cells were found to be more sensitive to Hsp90 inhibition than normal cells. Why that is has remained a matter of debate and is still unclear. In addition to increased Hsp90 dependence for some mutant cancer proteins and modifications of the Hsp90 machinery itself, a number of other characteristics of cancer cells probably contribute to this phenomenon; these include aneuploidy and overall increased numbers and levels of defective and mutant proteins, which all contribute to perturbed proteostasis. Work over the last two decades has demonstrated that many cancer-related proteins are Hsp90 clients, and yet only few of them have been extensively investigated, selected either on the basis of their obvious function as cancer drivers or because they proved to be convenient biomarkers for monitoring the effects of Hsp90 inhibitors. The purpose of our review is to go beyond these "usual suspects." We established a workflow to select poorly studied proteins that are related to cancer processes and qualify as Hsp90 clients. By discussing and taking a fresh look at these "unusual suspects," we hope to stimulate others to revisit them as novel therapeutic targets or diagnostic markers.

Toll-Like Receptors Promote Mitochondrial Translocation of Nuclear Transcription Factor Nuclear Factor of Activated T-Cells in Prolonged Microglial Activation

Microglia are resident macrophages in the CNS that scavenge pathogens, dying cells, and molecules using pattern recognition Toll-like receptors (TLRs). Nuclear factor of activated T-cells (NFAT) family transcription factors also regulate inflammatory responses in microglia. However, whether there exists cross talk between TLR and NFAT signaling is unclear. Here we show that chronic activation of murine microglia by prolonged stimulation of Toll-like receptor 4 (TLR4) ligand lipopolysaccharides (LPSs) leads to unexpected translocation of NFAT1 into mitochondria. This mitochondrial import of NFAT1 is independent of calcium/calcineurin signaling. Instead, inhibition of Toll/interleukin 1 receptor domain-containing adapter-inducing interferon-β (TRIF) pathway blocks the mitochondrial translocation of NFAT1. Functionally, inhibition of NFAT1 reduces the TRIF-mediated expression of interferon-β and compromises the production of ATP and reactive oxygen species in LPS-treated microglia. Therefore, our findings reveal a new inflammatory signaling pathway that links TLR with NFAT in regulating cytokine production and mitochondrial activity during chronic microglial activation.

SIGNIFICANCE STATEMENT

Nuclear factor of activated T-cells (NFAT) family transcription factors are known to undergo nuclear translocation in response to inflammatory stimulation. In this study, we uncovered a surprise transportation of NFATs into mitochondria in microglia after a prolonged treatment with bacteria endotoxin lipopolysaccharides (LPSs). LPSs activated Toll-like receptor 4 and its downstream Toll/interleukin 1 receptor-domain-containing adapter-inducing interferon-β (TRIF) to regulate the mitochondrial translocation of NFAT in microglia, whereas genetic inhibition of NFAT1 compromised TRIF-mediated cytokine production and reduced ATP and reactive oxygen species generation. These findings reveal a previously undescribed mitochondrial translocation of NFAT in microglia responding to extended activation of Toll-like receptor-mediated signaling transduction pathways.

Pathways to Parkinsonism Redux: convergent pathobiological mechanisms in genetics of Parkinson's disease

In the past few years, there have been a large number of genes identified that contribute to the lifetime risk of Parkinson's disease (PD). Some genes follow a Mendelian inheritance pattern, but others are risk factors for apparently sporadic PD. Here, we will focus on those genes nominated by genome-wide association studies (GWAS) in sporadic PD, with a particular emphasis on genes that overlap between familial and sporadic disease such as those encoding a-synuclein (SNCA), tau (MAPT), and leucine-rich repeat kinase 2 (LRRK2). We will advance the view that there are likely relationships between these genes that map not only to neuronal processes, but also to neuroinflammation. We will particularly discuss evidence for a role of PD proteins in microglial activation and regulation of the autophagy-lysosome system that is dependent on microtubule transport in neurons. Thus, there are at least two non-mutually exclusive pathways that include both non-cell-autonomous and cell-autonomous mechanisms in the PD brain. Collectively, these data have highlighted the amount of progress made in understanding PD and suggest ways forward to further dissect this disorder.

Receptor-Interacting Protein Kinase 3 Deficiency Delays Cutaneous Wound Healing

Wound healing consists of a complex, dynamic and overlapping process involving inflammation, proliferation and tissue remodeling. A better understanding of wound healing process at the molecular level is needed for the development of novel therapeutic strategies. Receptor-interacting protein kinase 3 (RIPK3) controls programmed necrosis in response to TNF-α during inflammation and has been shown to be highly induced during cutaneous wound repair. However, its role in wound healing remains to be demonstrated. To study this, we created dorsal cutaneous wounds on male wild-type (WT) and RIPK3-deficient (Ripk3^-/-) mice. Wound area was measured daily until day 14 post-wound and skin tissues were collected from wound sites at various days for analysis. The wound healing rate in Ripk3^-/- mice was slower than the WT mice over the 14-day course; especially, at day 7, the wound size in Ripk3^-/- mice was 53% larger than that of WT mice. H&E and Masson-Trichrome staining analysis showed impaired quality of wound closure in Ripk3^-/- wounds with delayed re-epithelialization and angiogenesis and defected granulation tissue formation and collagen deposition compared to WT. The neutrophil infiltration pattern was altered in Ripk3^-/- wounds with less neutrophils at day 1 and more neutrophils at day 3. This altered pattern was also reflected in the differential expression of IL-6, KC, IL-1β and TNF-α between WT and Ripk3^-/- wounds. MMP-9 protein expression was decreased with increased Timp-1 mRNA in the Ripk3^-/- wounds compared to WT. The microvascular density along with the intensity and timing of induction of proangiogenic growth factors VEGF and TGF-β1 were also decreased or delayed in the Ripk3^-/- wounds. Furthermore, mouse embryonic fibroblasts (MEFs) from Ripk3^-/- mice migrated less towards chemoattractants TGF-β1 and PDGF than MEFs from WT mice. These results clearly demonstrate that RIPK3 is an essential molecule to maintain the temporal manner of the normal progression of wound closure.

Regulation of tumour necrosis factor signalling: live or let die

Tumour necrosis factor (TNF) is a pro-inflammatory cytokine that has important roles in mammalian immunity and cellular homeostasis. Deregulation of TNF receptor (TNFR) signalling is associated with many inflammatory disorders, including various types of arthritis and inflammatory bowel disease, and targeting TNF has been an effective therapeutic strategy in these diseases. This Review focuses on the recent advances that have been made in understanding TNFR signalling and the consequences of its deregulation for cellular survival, apoptosis and regulated necrosis. We discuss how TNF-induced survival signals are distinguished from those that lead to cell death. Finally, we provide a brief overview of the role of TNF in inflammatory and autoimmune diseases, and we discuss up-to-date and future treatment strategies for these disorders.

A RIPK2 inhibitor delays NOD signalling events yet prevents inflammatory cytokine production

Intracellular nucleotide binding and oligomerization domain (NOD) receptors recognize antigens including bacterial peptidoglycans and initiate immune responses by triggering the production of pro-inflammatory cytokines through activating NF-κB and MAP kinases. Receptor interacting protein kinase 2 (RIPK2) is critical for NOD-mediated NF-κB activation and cytokine production. Here we develop and characterize a selective RIPK2 kinase inhibitor, WEHI-345, which delays RIPK2 ubiquitylation and NF-κB activation downstream of NOD engagement. Despite only delaying NF-κB activation on NOD stimulation, WEHI-345 prevents cytokine production in vitro and in vivo and ameliorates experimental autoimmune encephalomyelitis in mice. Our study highlights the importance of the kinase activity of RIPK2 for proper immune responses and demonstrates the therapeutic potential of inhibiting RIPK2 in NOD-driven inflammatory diseases.

A novel RIPK4-IRF6 connection is required to prevent epithelial fusions characteristic for popliteal pterygium syndromes

Receptor-interacting protein kinase 4 (RIPK4)-deficient mice have epidermal defects and fusion of all external orifices. These are similar to Bartsocas-Papas syndrome and popliteal pterygium syndrome (PPS) in humans, for which causative mutations have been documented in the RIPK4 and IRF6 (interferon regulatory factor 6) gene, respectively. Although genetically distinct, these syndromes share the anomalies of marked pterygia, syndactyly, clefting and hypoplastic genitalia. Despite the strong resemblance of these two syndromes, no molecular connection between the transcription factor IRF6 and the kinase RIPK4 was known and the mechanism underlying the phenotype was unclear. Here we describe that RIPK4 deficiency in mice causes epithelial fusions associated with abnormal periderm development and aberrant ectopic localization of E-cadherin on the apical membrane of the outer peridermal cell layers. In Xenopus, RIPK4 depletion causes the absence of ectodermal epiboly and concomitant gastrulation defects that phenocopy ectopic expression of dominant-negative IRF6. We found that IRF6 controls RIPK4 expression and that wild-type, but not kinase-dead, RIPK4 can complement the gastrulation defect in Xenopus caused by IRF6 malfunctioning. In contrast to the mouse, we observed only minor effects on cadherin membrane expression in Xenopus RIPK4 morphants. However, gastrulation defects were associated with a virtual absence of cortical actin in the ectodermal cells that face the blastocoel cavity and this was phenocopied in embryos expressing dominant-negative IRF6. A role for RIPK4 in actin cytoskeleton organization was also revealed in mouse epidermis and in human epithelial HaCaT cells. In conclusion, we showed that in mice RIPK4 is implicated in cortical actin organization and in E-cadherin localization or function, which can explain the characteristic epithelial fusions observed in PPSs. In addition, we provide a novel molecular link between IRF6 and RIPK4 that unifies the different PPSs to a common molecular pathway.

Discovery and preclinical profiling of 3-[4-(morpholin-4-yl)-7H-pyrrolo[2,3-d]pyrimidin-5-yl]benzonitrile (PF-06447475), a highly potent, selective, brain penetrant, and in vivo active LRRK2 kinase inhibitor

Leucine rich repeat kinase 2 (LRRK2) has been genetically linked to Parkinson's disease (PD) by genome-wide association studies (GWAS). The most common LRRK2 mutation, G2019S, which is relatively rare in the total population, gives rise to increased kinase activity. As such, LRRK2 kinase inhibitors are potentially useful in the treatment of PD. We herein disclose the discovery and optimization of a novel series of potent LRRK2 inhibitors, focusing on improving kinome selectivity using a surrogate crystallography approach. This resulted in the identification of 14 (PF-06447475), a highly potent, brain penetrant and selective LRRK2 inhibitor which has been further profiled in in vivo safety and pharmacodynamic studies.

Inflammation is genetically implicated in Parkinson's disease

Inflammation has long been associated with the pathogenesis of Parkinson's disease (PD) but the extent to which it is a cause or consequence is sill debated. Over the past decade a number of genes have been implicated in PD. Relatively rare missense mutations in genes such as LRRK2, Parkin, SNCA and PINK1 are causative for familial PD whereas more common variation in genes, including LRRK2, SNCA and GBA, comprise risk factors for sporadic PD. Determining how the function of these genes and the proteins they encode are altered in PD has become a priority, as results will likely provide much needed insights into contributing causes. Accumulating evidence indicates that many of these genes function in pathways that regulate aspects of immunity, particularly inflammation, suggesting close associations between PD and immune homeostasis.

PKK suppresses tumor growth and is decreased in squamous cell carcinoma of the skin

Non-melanoma skin cancer (NMSC) represents the most common cancer in the United States. Squamous cell carcinoma (SCC) of the skin is a sub-type of NMSC that shows a greater potential for invasion and metastasis. The current study identifies the Protein Kinase C-associated Kinase (PKK), which is also known as the Receptor-Interacting Protein Kinase 4 (RIPK4), as a suppressor of tumor growth in SCC of the skin. We show that expression of PKK is decreased in human SCC of the skin compared to normal skin. Further, suppression of PKK in human keratinocytes leads to increased cell proliferation. Use of RNA interference to reduce PKK expression in keratinocytes leads to an increase in S phase and in proteins that promote cell cycle progression. Consistent with the results obtained from cell culture, there is a dramatic increased tumorigenesis after PKK knockdown in a xenotransplant model and in soft agar assays. The loss of tumor suppression involves the NF-κB and p63 pathways. NF-κB is inhibited through inhibition of IKK function and there is increased nuclear TP63 activity after PKK knockdown. This study opens new avenues both in the discovery of disease pathogenesis and for potential treatments.

Ischemia-reperfusion injury of the retina is linked to necroptosis via the ERK1/2-RIP3 pathway

PURPOSE

Ischemia-reperfusion (IR) injury is involved in the pathology of many retinal disorders since it contributes to the death of retinal neurons and the subsequent decline in vision. We determined the molecular patterns of some of the principal molecules involved in necroptosis and investigated whether IR retinal injury is associated with the extracellular signal-regulated kinase-1/2- receptor-interacting protein kinase 3 (ERK1/2-RIP3) pathway.

METHODS

The cellular and subcellular localization of molecules involved in the cell death pathway, including RAGE, ERK1/2, FLIP, and RIP3, was determined with immunohistochemistry of cryosections of IR-injured retina from 2-month-old Long Evans rats. The total and phosphorylated protein levels were analyzed with quantitative western blots. ERK1/2 activity was inhibited by intravitreal injection of U0126, a highly selective inhibitor of mitogen-activated protein kinase 1/2 (MEK1/2).

RESULTS

The IR-injured rat retinas expressed two RAGE isoforms with different intracellular localizations at early time points after injury. At that time point, frame inhibition of ERK activation decreased RIP3 accumulation and cell death. FLIP was detected in the IR-injured rat retinas at early time points after ischemia reperfusion.

CONCLUSIONS

We report that the necroptotic cell death mechanism is executed by an ERK1/2-RIP3 pathway in the retinal ganglion cells in early stages after IR injury. Inhibition of ERK1/2 activity increased retinal ganglion cell (RGC) survival indicating that targeting of this pathway within the initial 12 h after IR injury can be used to inhibit the necroptosis pathway. We also provide evidence that a novel RAGE isoform is expressed in the early stages in rat retinal RGCs.

RIP kinases: key decision makers in cell death and innate immunity

Innate immunity represents the first line of defence against invading pathogens. It consists of an initial inflammatory response that recruits white blood cells to the site of infection in an effort to destroy and eliminate the pathogen. Some pathogens replicate within host cells, and cell death by apoptosis is an important effector mechanism to remove the replication niche for such microbes. However, some microbes have evolved evasive strategies to block apoptosis, and in these cases host cells may employ further countermeasures, including an inflammatory form of cell death know as necroptosis. This review aims to highlight the importance of the RIP kinase family in controlling these various defence strategies. RIP1 is initially discussed as a key component of death receptor signalling and in the context of dictating whether a cell triggers a pathway of pro-inflammatory gene expression or cell death by apoptosis. The molecular and functional interplay of RIP1 and RIP3 is described, especially with respect to mediating necroptosis and as key mediators of inflammation. The function of RIP2, with particular emphasis on its role in NOD signalling, is also explored. Special attention is given to emphasizing the physiological and pathophysiological contexts for these various functions of RIP kinases.

Toll signal transduction pathway in bivalves: complete cds of intermediate elements and related gene transcription levels in hemocytes of immune stimulated Mytilus galloprovincialis

Based on protein domain structure and organization deduced from mRNA contigs, 15 transcripts of the Toll signaling pathway have been identified in the bivalve, Mytilus galloprovincialis. Identical searches performed on publicly available Mytilus edulis ESTs revealed 11 transcripts, whereas searches performed in genomic and new transcriptome sequences of the Pacific oyster, Crassostrea gigas, identified 21 Toll-related transcripts. The remarkable molecular diversity of TRAF and IKK coding sequences of C. gigas, suggests that the sequence data inferred from Mytilus cDNAs may not be exhaustive. Most of the Toll pathway genes were constitutively and ubiquitously expressed in M. galloprovincialis, although at different levels, and clearly induced after in vivo injection with bacteria. Such over-transcription was more rapid and intense with Gram-negative than with Gram-positive bacteria. Injection of a fungus modulated the transcription of few Toll pathway genes, with the induction levels of TLR/MyD88 complex being always less intense. Purified LPS and β-glucans had marginal effect whereas peptidoglycans were ineffective. At the moment, we found no evidence of an IMD transcript in bivalves. In conclusion, mussels possess a complete Toll pathway which can be triggered either by Gram-positive or Gram-negative bacteria.

RIP3 overexpression sensitizes human breast cancer cells to parthenolide in vitro via intracellular ROS accumulation

AIM

Receptor-interacting protein 3 (RIP3) is involved in tumor necrosis factor receptor signaling, and results in NF-κB-mediated prosurvival signaling and programmed cell death. The aim of this study was to determine whether overexpression of the RIP3 gene could sensitize human breast cancer cells to parthenolide in vitro.

METHODS

The expression of RIP3 mRNA in human breast cancer cell lines (MCF-7, MDA-MB-231, MDA-MB-435 and T47D) was detected using RT-PCR. Both MDA-MB-231 and MCF-7 cells were transfected with RIP3 expression or blank vectors via lentivirus. Cell viability was measured with MTT assay; intracellular ROS level and cell apoptosis were analyzed using flow cytometry.

RESULTS

RIP3 mRNA expression was not detected in the four human breast cancer cell lines tested. However, the transfection induced higher levels of RIP3 protein in MCF-7 and MDA-MB-231 cells. Furthermore, overexpression of RIP3 decreased the IC50 values of parthenolide from 17.6 to 12.6 μmol/L in MCF-7 cells, and from 16.6 to 9.9 μmol/L in MDA-MB-231 cells. Moreover, overexpression of RIP3 significantly increased parthenolide-induced apoptosis and ROS accumulation in MCF-7 and MDA-MB-231 cells. Pretreatment with N-acetyl-cysteine abrogated the increased sensitivity of RIP3-transfected MCF-7 and MDA-MB-231 cells to parthenolide.

CONCLUSION

Overexpression of RIP3 sensitizes MCF-7 and MDA-MB-231 breast cancer cells to parthenolide in vitro via intracellular ROS accumulation.

Receptor-interacting protein kinase 2 promotes triple-negative breast cancer cell migration and invasion via activation of nuclear factor-kappaB and c-Jun N-terminal kinase pathways

INTRODUCTION

Metastasis is the main cause of breast cancer morbidity and mortality. Processes that allow for tumor cell migration and invasion are important therapeutic targets. Here we demonstrate that receptor-interacting protein kinase 2 (RIP2), a kinase known to be involved in inflammatory processes, also has novel roles in cancer cell migration and invasion.

METHODS

A total of six breast cancer expression databases, including The Cancer Genome Atlas, were assessed for RIP2 expression among various clinical subtypes and its role as a prognostic biomarker. mRNA fluorescence in situ hybridization (FISH) for RIP2 was performed on 17 stage III breast cancers to determine if there was a correlation between RIP2 expression and lymph node involvement. RNA-interference was used to knock-down RIP2 expression in MDA-MB-231, Htb126, SUM149PT, MCF7, T47D, and HCC1428 cells. Cell migration and invasion were measured in vitro by scratch/wound healing and transwell migration assays. A xenograft mouse model was used to assess tumor growth and chemosensitivity to docetaxel in vivo in MDA-MB-231 cells with and without RIP2 small hairpin RNA knockdown. Western blot and immunofluorescence imaging were used to evaluate protein expressions.

RESULTS

Interrogation of expression databases showed that RIP2 expression is significantly over-expressed in triple-negative breast cancers (TNBC: estrogen-receptor (ER) negative, progesterone-receptor (PR) negative, Her2/neu- (Her2) negative), compared to other clinical subtypes. High RIP2 expression correlates with worse progression-free survival using a combined breast cancer expression array dataset consisting of 946 patients. Multivariate analysis shows RIP2 as an independent prognostic biomarker. Knock-down of RIP2 significantly decreases migration in both scratch/wound healing and transwell migration assays in MDA-MB-231, Htb126, SUM149PT, MCF7, and T47D cells and is correlated with decreased Nuclear Factor-kappaB and c-Jun N-terminal kinase (JNK) activation. Finally, RIP2 knock-down leads to increased sensitivity to docetaxel and decreased tumor mass and lung metastases in a xenograft mouse model.

CONCLUSION

These results highlight RIP2 as a pro-metastasis kinase in patients with advanced breast cancer. These results also illustrate a novel role for this kinase in addition to its known role in inflammation, and suggest that targeting RIP2 may improve outcomes in advanced breast cancer patients, in which it is overexpressed.

Receptor-interacting protein 2 gene silencing attenuates allergic airway inflammation

Persistent activation of NF-κB has been associated with the development of asthma. Receptor-interacting protein 2 (Rip2) is a transcriptional product of NF-κB activation. It is an adaptor protein with serine/threonine kinase activity and has been shown to positively regulate NF-κB activity. We investigated potential protective effects of Rip2 gene silencing using small interfering RNA (siRNA) in an OVA-induced mouse asthma model. Rip2 protein level was found to be upregulated in allergic airway inflammation. A potent and selective Rip2 siRNA given intratracheally knocked down Rip2 expression in OVA-challenged lungs and reduced OVA-induced increases in total and eosinophil counts, and IL-4, IL-5, IL-13, IL-1β, IL-33, and eotaxin levels in bronchoalveolar lavage fluid. Rip2 silencing blocked OVA-induced inflammatory cell infiltration and mucus hypersecretion as observed in lung sections, and mRNA expression of ICAM-1, VCAM-1, E-selectin, RANTES, IL-17, IL-33, thymic stromal lymphopoietin, inducible NO synthase, and MUC5ac in lung tissues. In addition, elevation of serum OVA-specific IgE level in mouse asthma model was markedly suppressed by Rip2 siRNA, together with reduced IL-4, IL-5, and IL-13 production in lymph node cultures. Furthermore, Rip2 siRNA-treated mice produced significantly less airway hyperresponsiveness induced by methacholine. Mechanistically, Rip2 siRNA was found to enhance cytosolic level of IκBα and block p65 nuclear translocation and DNA-binding activity in lung tissues from OVA-challenged mice. Taken together, our findings clearly show that knockdown of Rip2 by gene silencing ameliorates experimental allergic airway inflammation, probably via interruption of NF-κB activity, confirming Rip2 a novel therapeutic target for the treatment of allergic asthma.

Daxx is a key downstream component of receptor interacting protein kinase 3 mediating retinal ischemic cell death

Receptor-interacting protein 3 (RIP3) has been implicated in ischemic necrosis of retinal cells. An in silico analysis followed by experimental validation identified death associated protein (Daxx) as a novel substrate of RIP3. In vitro binding studies revealed that RIP3 binds to the serine/proline/threonine-rich domain (amino acid 625-740) of Daxx. Upon ischemic insult, RIP3 phosphorylated Daxx at Ser-668 in the retinal ganglion cells, triggering nuclear export of Daxx. Depletion of RIP3 significantly inhibited nuclear export of Daxx and attenuated cell death to a great extent. Collectively, the findings of this study demonstrate that phosphorylation of Daxx by RIP3 comprises an important part of ischemic necrosis in rat retinal ganglion cells.

LRRK2 and autophagy: a common pathway for disease

LRRK2 (leucine-rich repeat kinase 2) is an enzyme implicated in human disease, containing kinase and GTPase functions within the same multidomain open reading frame. Dominant mutations in the LRRK2 gene are the most common cause of familial PD (Parkinson's disease). Additionally, in genome-wide association studies, the LRRK2 locus has been linked to risk of PD, Crohn's disease and leprosy, and LRRK2 has also been linked with cancer. Despite its association with human disease, very little is known about its pathophysiology. Recent reports suggest a functional association between LRRK2 and autophagy. Implications of this set of data for our understanding of LRRK2′s role in physiology and disease are discussed in the present paper.

An emerging role for LRRK2 in the immune system

Missense mutations in LRRK2 (leucine-rich repeat kinase 2) contribute significantly to autosomal dominant PD (Parkinson's disease). Genome-wide association studies have suggested further that mutations in LRRK2 comprise a risk factor for sporadic PD. How LRRK2 contributes to PD, however, is largely unknown. Recent work has shown that LRRK2 is highly expressed in tissue and circulating immune cells and is suggestive of a potential role for LRRK2 in innate immunity. These studies and their potential implications for PD are discussed in the present paper.

RIP3 expression induces a death profile change in U2OS osteosarcoma cells after 5-ALA-PDT

BACKGROUND AND OBJECTIVE

The receptor-interacting protein 3 (RIP3) has recently been outlined as a key necrosis mediator but is also thought to participate in the regulation of apoptosis. The aim of this study is to compare the cell death profile induced by 5-aminolevulic acid (5-ALA)-mediated photodynamic therapy (PDT) in the RIP3-deficient cell line U2OS and in U2OS cells in which the expression of RIP3 was restored.

MATERIALS AND METHODS

RIP3-expressing U2OS cells (RIP3-U2OS) were obtained after transfection and antibiotic selection. Wild type and RIP3-U2OS cells were treated by 5-ALA-PDT. Overall cell viability was evaluated and different parameters characteristic of apoptosis, autophagy, and necrosis were studied.

RESULTS

Surprisingly, the survival of RIP3-U2OS cells was higher compared to that of the wild type cells. In addition, RIP3-U2OS cell death was decreased by a zVAD-fmk pre-treatment. A higher cleavage of caspase-3, 7, 8, 9, and PARP was also detected in these cells, pointing out to the activation of caspase-dependent apoptosis. In parallel, a thrust of autophagy was clearly identified in the RIP3-U2OS cells. Conversely, RIP3-U2OS exhibited a lower level of necrosis than the wild types. Interestingly, necrostatin-1 efficiently decreased necrosis level in RIP3-U2OS but not in wild type cells.

CONCLUSION

Expression of RIP3 in U2OS cells led to a better survival but also to a death profile change in response to PDT. The apoptotic and autophagic pathways were clearly up-regulated compared to the RIP3-deficient wild type cells. However, induction of necrosis was weaker in the RIP3-U2OS cells. In this context, autophagy is likely to play a protective role against PDT-induced cell death and to allow a better survival of RIP3-U2OS cells. This work also highlights the important role played by RIP3 in the apoptotic pathway, although the modalities are still widely unknown.

Pathogen-mediated proteolysis of the cell death regulator RIPK1 and the host defense modulator RIPK2 in human aortic endothelial cells

Porphyromonas gingivalis is the primary etiologic agent of periodontal disease that is associated with other human chronic inflammatory diseases, including atherosclerosis. The ability of P. gingivalis to invade and persist within human aortic endothelial cells (HAEC) has been postulated to contribute to a low to moderate chronic state of inflammation, although how this is specifically achieved has not been well defined. In this study, we demonstrate that P. gingivalis infection of HAEC resulted in the rapid cleavage of receptor interacting protein 1 (RIPK1), a mediator of tumor necrosis factor (TNF) receptor-1 (TNF-R1)-induced cell activation or death, and RIPK2, a key mediator of both innate immune signaling and adaptive immunity. The cleavage of RIPK1 or RIPK2 was not observed in cells treated with apoptotic stimuli, or cells stimulated with agonists to TNF-R1, nucleotide oligomerization domain receptor 1(NOD1), NOD2, Toll-like receptor 2 (TLR2) or TLR4. P. gingivalis-induced cleavage of RIPK1 and RIPK2 was inhibited in the presence of a lysine-specific gingipain (Kgp) inhibitor. RIPK1 and RIPK2 cleavage was not observed in HAEC treated with an isogenic mutant deficient in the lysine-specific gingipain, confirming a role for Kgp in the cleavage of RIPK1 and RIPK2. Similar proteolysis of poly (ADP-ribose) polymerase (PARP) was observed. We also demonstrated direct proteolysis of RIPK2 by P. gingivalis in a cell-free system which was abrogated in the presence of a Kgp-specific protease inhibitor. Our studies thus reveal an important role for pathogen-mediated modification of cellular kinases as a potential strategy for bacterial persistence within target host cells, which is associated with low-grade chronic inflammation, a hallmark of pathogen-mediated chronic inflammatory disorders.

NF-κB, the first quarter-century: remarkable progress and outstanding questions

The ability to sense and adjust to the environment is crucial to life. For multicellular organisms, the ability to respond to external changes is essential not only for survival but also for normal development and physiology. Although signaling events can directly modify cellular function, typically signaling acts to alter transcriptional responses to generate both transient and sustained changes. Rapid, but transient, changes in gene expression are mediated by inducible transcription factors such as NF-κB. For the past 25 years, NF-κB has served as a paradigm for inducible transcription factors and has provided numerous insights into how signaling events influence gene expression and physiology. Since its discovery as a regulator of expression of the κ light chain gene in B cells, research on NF-κB continues to yield new insights into fundamental cellular processes. Advances in understanding the mechanisms that regulate NF-κB have been accompanied by progress in elucidating the biological significance of this transcription factor in various physiological processes. NF-κB likely plays the most prominent role in the development and function of the immune system and, not surprisingly, when dysregulated, contributes to the pathophysiology of inflammatory disease. As our appreciation of the fundamental role of inflammation in disease pathogenesis has increased, so too has the importance of NF-κB as a key regulatory molecule gained progressively greater significance. However, despite the tremendous progress that has been made in understanding the regulation of NF-κB, there is much that remains to be understood. In this review, we highlight both the progress that has been made and the fundamental questions that remain unanswered after 25 years of study.

Necroptosis, necrostatins and tissue injury

Cell death is an integral part of the life of an organism being necessary for the maintenance of organs and tissues. If, however, cell death is allowed to proceed unrestricted, tissue damage and degenerative disease may ensue. Until recently, three morphologically distinct types of cell death were recognized, apoptosis (type I), autophagy (type II) and necrosis (type III). Apoptosis is a highly regulated, genetically determined mechanism designed to dismantle cells systematically (e.g. cells that are no longer functionally viable), via protease (caspase) action, and maintain homeostasis. Autophagy is responsible for the degradation of cytoplasmic material, e.g. proteins and organelles, through autophagosome formation and subsequent proteolytic degradation by lysosomes, and is normally considered in the context of survival although it is sometimes associated with cell death. Necrosis was formerly considered to be an accidental, unregulated form of cell death resulting from excessive stress, although it has been suggested that this is an over-simplistic view as necrosis may under certain circumstances involve the mobilization of specific transduction mechanisms. Indeed, recently, an alternative death pathway, termed necroptosis, was delineated and proposed as a form of ‘programmed necrosis’. Identified with the aid of specific inhibitors called necrostatins, necroptosis shares characteristics with both necrosis and apoptosis. Necroptosis involves Fas/tumour necrosis factor-α death domain receptor activation and inhibition of receptor-interacting protein I kinase, and it has been suggested that it may contribute to the development of neurological and myocardial diseases. Significantly, necrostatin-like drugs have been mooted as possible future therapeutic agents for the treatment of degenerative conditions.

Programmed necrosis: backup to and competitor with apoptosis in the immune system

Programmed cell death is essential for the development and maintenance of the immune system and its responses to exogenous and endogenous stimuli. Studies have demonstrated that in addition to caspase-dependent apoptosis, necrosis dependent on the kinases RIP1 and RIP3 (also called necroptosis) is a major programmed cell-death pathway in development and immunity. These two programmed cell-death pathways may suppress each other, and necroptosis also serves as an alternative when caspase-dependent apoptosis is inhibited or absent. Here we summarize recent advancements that have identified the molecular mechanisms that underlie necroptosis and explore the mechanisms that regulate the interplay between apoptosis and necroptosis.

5-ALA-PDT induces RIP3-dependent necrosis in glioblastoma

Glioblastoma constitute the most frequent and deadliest brain tumors of astrocytic origin. They are resistant to all current therapies and are associated with a high rate of recurrence. Glioblastoma were previously shown to respond to treatments by 5-aminolevulinic acid (5-ALA)-based photodynamic therapy (PDT) mainly by activating a necrotic type of cell death. The receptor-interacting protein 3 (RIP3) has recently been outlined as a key mediator of this caspase-independent form of programmed cell death. In the present study, we analyzed the necrotic mechanism induced by 5-ALA-PDT in human glioblastoma cells and explored the role of RIP3 in this context. Our results show that PDT-induced necrosis is dependent on RIP3, which forms aggregates and colocalizes with RIP1 following photosensitization. We demonstrate that PDT-mediated singlet oxygen production is the cause of RIP3-dependent necrotic pathway activation. We also prove that PDT induces the formation of a pro-necrotic complex containing RIP3 and RIP1 but lacking caspase-8 and FADD, two proteins usually part of the necrosome when TNF-α is used as a stimulus. Thus, we hypothesize that PDT might lead to the formation of a different necrosome whose components, besides RIP1 and RIP3, are still unknown. In most cases, glioblastoma are characterized by a constitutive activation of NF-κB. This factor is a key regulator of various processes, such as inflammation, immune response, cell growth or apoptosis. Its inhibition was shown to further sensitize glioblastoma cells to PDT-induced necrosis, however, no difference in RIP3 upshift or aggregation could be observed when NF-κB was inhibited.

cIAP1/2 are direct E3 ligases conjugating diverse types of ubiquitin chains to receptor interacting proteins kinases 1 to 4 (RIP1-4)

The RIP kinases have emerged as essential mediators of cellular stress that integrate both extracellular stimuli emanating from various cell-surface receptors and signals coming from intracellular pattern recognition receptors. The molecular mechanisms regulating the ability of the RIP proteins to transduce the stress signals remain poorly understood, but seem to rely only partially on their kinase activities. Recent studies on RIP1 and RIP2 have highlighted the importance of ubiquitination as a key process regulating their capacity to activate downstream signaling pathways. In this study, we found that XIAP, cIAP1 and cIAP2 not only directly bind to RIP1 and RIP2 but also to RIP3 and RIP4. We show that cIAP1 and cIAP2 are direct E3 ubiquitin ligases for all four RIP proteins and that cIAP1 is capable of conjugating the RIPs with diverse types of ubiquitin chains, including linear chains. Consistently, we show that repressing cIAP1/2 levels affects the activation of NF-κB that is dependent on RIP1, -2, -3 and -4. Finally, we identified Lys51 and Lys145 of RIP4 as two critical residues for cIAP1-mediated ubiquitination and NF-κB activation.